CHAPTER III

THE CLASSIFICATION OF CRUSTACEA

_Table of Classification of Crustacea_

CLASS CRUSTACEA.

Subclass BRANCHIOPODA - {Order Anostraca. { " Notostraca. { " Conchostraca. { " Cladocera.

" OSTRACODA - { " Myodocopa. { " Podocopa.

" COPEPODA - { " Eucopepoda. { " Branchiura.

" CIRRIPEDIA - { " Thoracica. { " Rhizocephala.

" MALACOSTRACA.

Series _LEPTOSTRACA_ - " Nebaliacea.

" _EUMALACOSTRACA._

Division _Syncarida_ - " Anaspidacea.

" _Peracarida_ - { " Mysidacea. { " Cumacea. { " Tanaidacea. { " Isopoda. { " Amphipoda.

" _Eucarida_ - { " Euphausiacea. { " Decapoda.

" _Hoplocarida_ - " Stomatopoda.

Occasionally there may be found in rain-water puddles and the like, in the South of England, a beautiful, transparent, shrimp-like animal, an inch or more in length, to which the name of "Fairy Shrimp" has been given (Fig. 10). It is known in technical zoology as _Chirocephalus diaphanus_, and is a representative of the subclass BRANCHIOPODA. The members of this group are distinguished from other Crustacea by their flattened, leaf-like feet, each of which is divided into a number of lobes, and has a gill plate on the outer side. In _Chirocephalus_ there is no carapace, and the head is followed by eleven distinct body segments, each bearing a pair of leaf-like, or rather fin-like, feet. The hinder part of the body has no appendages, and ends in a forked tail. In the female a large pouch hangs from the under-side of the body, just behind the limb-bearing part, and is often found filled with eggs. In the male, a pair of remarkable-looking appendages, each shaped somewhat like a hand with webbed fingers, hang in front of the head. These are connected with the antennæ, and are known as the "claspers," from their function in seizing and holding the female. The eyes are set on movable stalks. Those Branchiopoda which, like _Chirocephalus_, have no carapace, form the order ANOSTRACA.

A second order, the NOTOSTRACA, is represented by _Apus cancriformis_ (Plate II.), which occurs in many places in Europe in ponds and puddles, and very rarely indeed in Britain. In _Apus_ there is a large dorsal shield, or carapace, covering the greater part of the body, which consists of a large number of segments (about twenty-eight), and ends behind in a pair of long antenna-like filaments. The fin-like feet are also very numerous (about sixty-three pairs). The eyes are not stalked, but are set close together on the upper surface of the carapace.

The third order of the Branchiopoda, the CONCHOSTRACA (Fig. 11), are not represented in Britain, though several species occur on the Continent of Europe. In these the carapace forms a bivalved shell, completely enclosing the body and limbs, and closely resembling that of a small Mollusc.

The fourth order, the CLADOCERA, comprises the so-called "Water-fleas," which are abundant everywhere in ponds and lakes (Fig. 12). They are all of small size, almost or quite microscopic. The carapace, as in the Conchostraca, forms a bivalved shell, but does not enclose the head. There is a single large eye, which really corresponds to two eyes fused together. A pair of large antennæ, each with two branches, carrying long feathered hairs, project at the sides of the head, and are used in swimming with a peculiar jumping motion, from which the popular name of the animals is derived. There are not more than six pairs of feet. The "Water-fleas," of which _Daphnia pulex_ is one of the commonest species, are very beautiful and interesting objects for microscopic examination, on account of their transparency, which allows many details of their internal structure to be studied in the living animal.

The OSTRACODA (Fig. 13), which form the second subclass in the system of classification here adopted, are nearly all microscopic animals, and are found abundantly in fresh water as well as in the sea. The carapace forms a bivalved shell, which completely encloses the body and limbs, and is often sculptured in an elegant fashion. The Ostracoda are remarkable for the very small number of their appendages. There are not more than two pairs of limbs behind the maxilla. Most of the species are included in two orders, the _Myodocopa_ and the _Podocopa_, of which the former may generally be distinguished by a notch in the anterior part of the margin of the shell (Fig. 13, A, _n_). In the _Podocopa_ the margin is entire.

The subclass COPEPODA comprises animals, for the most part of microscopic size, which are abundant in fresh water and in the sea. The common fresh-water genus _Cyclops_ (Fig. 14) furnishes a good example of the type of structure characteristic of the class. The body is somewhat pear-shaped, with a narrow abdomen ending in a "caudal fork." The body is divided into somites, and there is no overlapping carapace, although the head and the first two thoracic somites are coalesced. There are four pairs of two-branched, oar-like, swimming feet, and a fifth pair, found in some other Copepoda, is represented in _Cyclops_ by minute vestiges on the first segment of the narrow posterior part of the body. The antennules are very large, unbranched and composed of numerous segments; the antennæ are much smaller. In addition to the usual mandibles, maxillulæ, and maxillæ, there is a pair of maxillipeds which really represent the first pair of trunk limbs. There is a single red eye in the middle of the front of the head. This eye is not formed, like the single eye of the Cladocera, by fusion of a pair of eyes, but it corresponds to a median eye of simple structure which is found in the Branchiopoda, Ostracoda, and many other Crustacea, in addition to the paired compound eyes. From the fact that this median eye is the only one present in the earliest larval stage of Crustacea, the Nauplius (see