Part 2

Abdominal segments with basal transverse creeping welts or areas of microscopic points. The massive compact head-capsule with the præfrontal sclerite large, distinct, the externo-lateral plates large, mussel-shaped and very thin. The mentum is not completely divided medially. The maxillæ are large and of primitive structure, the cardines and stipites distinct, the two distal lobes large, subequal in size, covered with hairs and bearing sensory organs. Mandibles with one or more dorsal and two or more ventral teeth in addition to the apical point.

The differences between these allied divisions are best indicated by a key.

LARVAE

1. Spiracular disk with only the two long ventral lobes remaining; spiracles lacking or vestigial; abdominal segments with both dorsal and ventral welts; strictly aquatic. _Antocharia._ Spiracular disk surrounded by four or five short lobes; spiracles large and functional; abdominal segments with ventral welts only (except the dorsum of segment eight); terrestrial or semiaquatic.

2. Body moderately elongated and covered with a long dark pubescence; spiracular disk squarely truncated, surrounded by five subequal stout lobes; mentum with five subequal teeth, the lateral one of either side not conspicuously reduced. _Rhamphidaria._

Body very long and slender, glabrous; spiracular disk obliquely truncated, surrounded by four slender naked lobes; mentum with three subequal primary teeth and a much reduced lateral tooth on either side. _Dicranoptycharia._

PUPAE

1. Pronotal breathing-horns branched; aquatic. _Antocharia._

Pronotal breathing-horns not branched; semiaquatic or terrestrial.

2. Pronotal breathing-horns distinct, elongate-cylindrical. _Rhamphidaria._

Pronotal breathing-horns apparently lacking, microscopic. _Dicranoptycharia._

THE SUBTRIBE DICRANOPTYCHA

A Key to the Species of Dicranoptycha

LARVAE

1. Spiracular disk with the dark markings less extensive; the mark of the lateral lobes not contiguous with the spiracle or the triangular area on the disk; dorsal marking indistinct or lacking. _D. winnemana_ Alex.

Spiracular disk with the dark markings more extensive; the mark of the lateral lobes suffusing the ventral inner margin of the spiracle and usually closely approximated or nearly contiguous with the triangular area on the disk; dorsal marking black, transversely rectangular. _D. minima_ Alex.

Description of the Species.

DESCRIPTION OF THE SPECIES

1916 _Dicranoptycha winnemana_ Alexander; Proc. Acad. Nat. Sci. Phila., pp. 500, 501; Pl. 25, fig. 12.

_Larva._--Length, 20-22 mm. Diameter, 0.9-1.1 mm.

Coloration varying from white to almost black depending on the nature and amount of the food eaten which shows clearly through the transparent integument. The fat-bodies likewise show through and give a white color to the larva especially after death.

Form very elongate (fig. 1), body terete; integument very glabrous, transparent and glassy. Prothoracic segment a little longer than the mesothorax which, in turn, slightly exceeds the metathorax. The intermediate abdominal segments are elongated. The basal ring of sternites two to eight bears a transverse band or area of microscopic chitinized spicules, the one on the eighth segment split lengthwise by a capillary line. A similar band occurs in the same position on the dorsum of the eighth segment but the pleural region is devoid of such a band.

Spiracular disk (fig. 8) moderate in size, obliquely truncated, surrounded by four lobes, a pair of small, slender, lateral lobes and short, broader ventral lobes. The usual dorso-median lobe is lacking but its position is indicated by a gently rounded convexity. The inner face of the lateral lobe bears a narrow semi-lunate black mark with the concavity toward the spiracle, the proximal end acutely pointed. The ventral lobes bear a similar but smaller subrectangular black mark. A pale and usually indistinct dusky mark occupies the inner face of the dorsal lobe. On the disk between, and slightly below the level of, the spiracles is a large brown triangular or V-shaped mark. The spiracles are small, separated from one another by a distance equal to about 2.5 to 3 times the diameter of one; the center-piece of the spiracle is black, the ring yellow surrounded by an outer dusky margin. Anal gills fleshy and protuberant as a blunt ring surrounding the anus (fig. 10).

Head-capsule (fig. 2) of the compact, massive type of the Limnobiini; præfrontal sclerite (fig. 3) large and distinct; the sclerite broad with the sides subparallel to about midlength, thence tapering gradually to the tip which is entire; there are two or three punctures at the margin before midlength. Interno-lateral plates narrow, a little longer than the præfrons; externo-lateral plates very broad, thin and flattened with the posterior margin very obtuse and the inner ventral portions continuous with the mental plate. Labrum (fig. 3) very broad and extensive, flattened, pale in color, the anterior margin with about two sense-organs. Mentum (fig. 4) deeply split behind but not completely divided, the anterior margin with three primary teeth that are subequal in size or the middle one a little smaller; a much reduced lateral tooth on either side. Præmentum smaller than the hypopharynx, in outline roughly oval or semicircular with the two labial palpi surrounded by hairs at the base. Hypopharynx (fig. 5) consisting of two chitinized arms that are contiguous but not fused medially, the concavity between them filled with a rounded cushion that is covered with tubercles arranged in more or less distinct oblique parallel rows. Antennæ (fig. 6) two-segmented, the basal segment cylindrical with an auditory plate on the face at beyond midlength; apical segment long and slender, in length but slightly less than the basal segment, tapering gradually to the bluntly rounded apex. Mandibles (fig. 7) simple with the teeth blunt; apical point longer than the lateral teeth; dorsal tooth single, broad, very flattened and obtusely pointed; ventral teeth two, a little smaller than the dorsal tooth. Maxillæ (fig. 2) of a generalized structure, the cardines distinct and feebly chitinized; distal lobes of the organ consisting of a subequal inner and outer lobe; the outer lobe with an abundance of long, delicate hairs and bearing a few sensory papillæ including one larger palpiform organ.

_Pupa._--Length, 9.1-12.8 mm. Width, d.-s., 1.6-1.8 mm. Depth, d.-v., 1.6-1.9 mm.

Thoracic dorsum shiny light brown; in very old pupæ the color is much darker, but still retains a much brighter color than the leg and wing-sheaths; abdomen pale becoming darker in age, especially on the pleura.

Cephalic crest (fig. 13) low and depressed, inconspicuous, lying between the antennal bases which extend beyond it; there are four small setigerous lobes, the larger pair of which are posterior in position. Front between the eyes broad, subparallel. Two blunt tubercles on either side of the forehead. Eyes large, with coarse ommatidia. Labrum semicircular in outline, tumid. Labial lobes large, oval, contiguous with one another, at the tip of the labrum. Maxillary palpi moderately long and slender, nearly straight, gradually narrowed to the tip which ends opposite the knee-joint of the fore legs. Antennæ with the basal segments separated only by the cephalic crest, the sheaths ending about opposite or a little before the lateral angle of the thorax.

Pronotal breathing-horns (fig. 14) very small, almost microscopic; when viewed from the dorsal aspect appearing as tiny triangular tubercles. Mesonotum moderately convex, unarmed, the V-shaped suture distinct; a few setæ on the mesonotum, including one near the end of each scutal lobe. Wing-sheaths rather short, but narrow, ending about opposite midlength of the third abdominal segment. Leg-sheaths ending opposite the base of the fifth abdominal segment, the tips of the tarsi ending about on a common level or those of the fore legs a trifle longer.

Abdominal segments (fig. 11) subdivided into four annuli that bear transverse bands of microscopic setæ; these bands increase in width from the basal to the apical. Spiracles on the pleural region of segments two to seven, lying opposite the third annulus and close to the ventral margin of the pleura. No spiracles are discernible on the dorsum of the eighth segment. Male cauda (fig. 11) with the ventral lobes very blunt, rounded; the dorsal lobes very small, terminating in a sharp spine that is directed dorsad and bears a weak seta near its base. Female cauda (fig. 12) with the ventral lobes a little longer than the dorsal lobes; the latter at the outer angle of the apex with a short stout spine that is directed dorsad as in the male.

_Nepionotype_ (type larva), Lawrence, Kansas, April 2, 1918.

_Neanotype_ (type pupa), with the type larva, May 6, 1918.

_Paratypes_, larvæ and pupæ, about fifty from the type locality, March 20 to May 20, 1918.

_Dicranoptycha minima_ Alexander.

1919 _Dicranoptycha minima_ Alexander; Ent. News, Vol. 30.

The larva is very similar to that of _D. winnemana_ as described above, but is slightly smaller. The spiracular disk (fig. 9) has the dark markings much more extensive. The mark of the lateral lobes is contiguous with the spiracles and is also closely approximated to the large triangular brown mark on the disk. There is a large transverse rectangular mark occupying the inner face of the dorsal lobe. The marking of the ventral lobe is about as in _D. winnemana_.

_Nepionotype_, Lawrence, Kansas, July 11, 1918.

_Neanotype_, Lawrence, Kansas, July 21, 1918.

_Paratypes_, a few larvæ from the type-locality.

Explanation of the Figures

A--Labial Lobes; E--Eye; EL--Externo-lateral Plate; G--Anal Gills; IL--Interno-lateral Plate; Lb--Labrum; M--Maxillary Palpus; P--Pronotal Breathing-horn; Pf--Præfrons; S--Spiracle.

Fig. 1. Larva of _Dicranoptycha winnemana_, ventral aspect of body.

Fig. 2. The same, head-capsule, ventral aspect.

Fig. 3. The same, head-capsule, dorsal aspect.

Fig. 4. The same, mentum, ventral aspect.

Fig. 5. The same, hypopharynx, ventral aspect.

Fig. 6. The same, antenna.

Fig. 7. The same, mandible.

Fig. 8. Larva of _Dicranoptycha winnemana_, spiracular disk, dorso-caudal aspect.

Fig. 9. Larva of _D. minima_, spiracular disk, caudal aspect, the anal gills protruded.

Fig. 10. Larva of _D. winnemana_, spiracular disk, lateral aspect.

Fig. 11. Pupa of _D. winnemana_, lateral aspect of male.

Fig. 12. The same, lateral aspect of female cauda.

Fig. 13. The same, head and mouth-parts, ventral aspect.

Fig. 14. The same, pronotal breathing-horn, enlarged.

The Central Nervous System of Nucula and Malletia

WILLIAM A. HILTON

These bivalve forms are grouped among the simplest of the molloscs. It is especially from the condition in _Nucula_ as described by Pelseneer '91, that the conception of the most anterior ganglion being composed of four ganglia, has its chief support. Drew '01, who has also studied _Nucula_, believes that the lobes of the ganglion in _Nucula_ are superficial and that the four connectives coming from the ganglion may be interpreted in another way. That is, that one pair of nerves may represent an otocystic branch partly fused with the connective. This view seemed reasonable to him as Stempel '99 in _Solenyma_ found the otocystic nerves arose directly from the cerebral ganglion.

The two species of this group used for study were collected at Laguna Beach. _Nucula castrensis_ Hinds, occurs abundantly at low tide under rocks. It is rather small for dissection, but very good complete series were obtained and stained in hematoxylin. _Malletia faba_ Dall, was much less abundant. Specimens were obtained from holdfasts or from dredging. Although this was a larger species, gross dissection was not very easily carried out on any of the specimens, but good series were made.

The ganglia of _Nucula_ are easily studied in section. The cerebral mass seems composed of one main mass, partly divided into four subdivisions, the two central most completely fused, and the lateral quite distinct in places. The central portion might represent the cerebral ganglia and the lateral, the pleural if we take that interpretation. The pedal ganglion is made of right and left parts quite completely fused except at the margins. The pedal mass is the smallest of the three chief ganglionic areas. The visceral ganglia are quite widely separated and a little larger than the pedal mass.

The ganglia of _Malletia_ are in general plan similar to those of _Nucula_, the greatest differences being in the cerebral mass. The cerebro-pleural mass seems almost one. In most sections it is very compact and a little more complicated in structure than the ganglion of _Nucula_. However there are two small ventral ganglionic branches or small ganglia attached to the ventral side of the cerebral mass. These small ganglia may represent the visceral. Farther back in a cross section series as the cerebral mass disappears two other small branches take origin and run parallel to the nerves from the ganglionic cords. These two branches on each side seem to run together before the pedal ganglia are reached. Neither of these pairs of nerves seems connected with an otocyst.

At the cephalic end of the cerebro-pleural ganglion the large ganglionic cords are in evidence. A little distance from the cephalic end on the dorsal side there are quite large groups of cells down from the surface and surrounded by nerve fibers. The course of the fibers here is quite complex. On the ventral lateral sides of the ganglia are paired light areas of fibers which may be traced into the fibers of the ganglionic cords.

The pedal ganglion is small and much as in _Nucula_. The visceral ganglia are larger and widely separated.

In both _Nucula_ and _Malletia_ young specimens were used for study. In _Nucula_ there was more the appearance of four ganglia in the cerebro-pleural mass, and the ganglia seem less complex than in _Malletia_. This last species has more separate pleural ganglia, if the ganglionic cords can be so regarded.

In neither of the species studied were all parts of the connectives easy to follow, so it was impossible to test the suggestions of Drew, but in both species there is some indication of two lateral lobes of the cerebral mass, and in _Nucula_ there is good evidence of two central ganglia as well as the smaller lateral ones. The lateral ganglia of the cerebral mass are most clearly separated in _Malletia_. In _Nucula_ the lateral ganglia are larger in proportion and the distribution of the gray and white matter is more irregular.

REFERENCES

_Drew, G. A._ 1901

The life history of Nucula delphinodonta. Quart, jour. sc. vol. 44, pt. 3.

_Pelseneer, P._ 1891

Contribution á l'étude des Lamellibranchs. Arch. d. biol. xi.

_Stempell_ 1899

Zur Anatomie von Solrmya togata. Zool. Jahrb. Bd. xiii. (_Contribution from the Zoological Laboratory of Pomona College_)

EXPLANATION OF FIGURES

Fig. 1. Diagram of the ganglia of _Nucula castrensis_, reconstructed from serial sections. The probable position of the connectives is shown and the proportionate distances between ganglia are given. The upper ganglion is the cerebro-pleural with large nerves leading off from the ganglion which is itself lobed into four chief lobes. The pedal ganglion is next. In section the pedal ganglion at one place seems to be made up of four parts which may correspond to four connectives from the cerebro-pleural although only one pair of connectives was clearly determined. The visceral ganglion is connected with the pedal below. ×70.

Fig. 2. Cross section of cerebro-pleural ganglion. On the right side one of the lateral ganglia is shown. The one of the other side does not show because the section is not straight across. The dorsal side is up. ×300.

Fig. 3. Section of the pedal mass of _Nucula_, through the center. The dorsal side is up. ×300.

Fig. 4. Left side of the visceral mass of _Nucula_. Dorsal side up. ×300.

Fig. 5. Nerve cells from the central nervous system of _Nucula_. ×450.

Fig. 6. Section through the body of _Nucula_ showing the position of the cerebro-pleural ganglion cut through the center. Dorsal side up. The cellular portion of the ganglion is black. ×70.

Fig. 7. Section through the body of _Nucula_ at the level of the visceral nerves which are shown on either side of the section. The area of nerve cells is shown in black. ×70.

Fig. 8. Reconstruction from serial sections of the cerebro-pleural mass nerves and connectives of _Malletia faba_. The drawing is a ventral view, the cephalic side is at the top. ×70.

Fig. 9. Reconstruction of pedal ganglion of _Malletia_ from the ventral side. Cephalic side at the top. ×70.

Fig. 10. Reconstruction of visceral ganglia of _Malletia_. ×70.

Fig. 11. Section through cerebro-pleural mass of _Malletia_. The dorsal side is up. On the ventral side to the left and right are the beginnings of the lateral lobes or ganglionic cords which may represent the pleural ganglia. In this species the cerebral ganglia are not separated into right and left halves as in _Nucula_. ×300.

Fig. 12. Section through the central part of the pedal mass of _Malletia_. The dorsal side is up. ×300.

Fig. 13. Section through one visceral ganglion of _Malletia_. The dorsal side is up. ×300.

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