302 COMPARATIVE ANATOMY CHAP. 



order to associate the limbs of these segments with the limbs already 

 present on the head in the work of taking in food. 



3. To the fact that the extremities in certain regions of the body 

 give up their various functions (which are almost always associated 

 with movement), and consequently become reduced or quite disappear. 

 The independence of the segments which carry such reduced limbs 

 is then more or less lost, that is if the said region does not in any 

 other way gain a locomotory significance. 



4. To the loss of the capacity for active locomotion by adapta- 

 tion to the parasitic mode of life ; this adaptation leads to the reduc- 

 tion of the extremities and the more or less complete obliteration of 

 the metamerism of the whole body. 



Having prefaced these remarks by way of elucidation, we proceed 

 to sketch in outline the external morphology of the body in the prin- 

 cipal groups of the Crustacea. 



I. Entomostraca. 



Phyllopoda. In Branchipus (Fig. 191) the trunk appears distinctly segmented, 

 and falls into an anterior and a posterior region which are called thorax and abdomen 

 respectively. The 11 segments of the thorax carry 11 pairs of limbs ; the abdomen, 

 which consists of many segments, is limbless, and ends in 2 so-called furcal plates. 

 A shield or shell fold is wanting. The head carries 2 stalked movable lateral 

 eyes. In Apus the body is covered for the most part with a flat shell fold, not fusing 

 with any of the anterior thoracic segments. The trunk consists of a large number of 

 segments, the most posterior being limbless. In the Estlieridce the body is quite 

 enveloped in a bivalve shell, which also covers the limbs laterally. The trunk 

 consists of numerous segments. The posterior segments, which have no limbs, form 

 a short abdomen, as opposed to the thorax of many segments. A bivalve shell also 

 occurs in the Cladocera ; here also the shell covers the limbs. In this case, however, 

 it leaves the well-marked head uncovered. In the trunk the segmentation is 

 obliterated. It carries 4 to 6 pairs of limbs, and ends in an unsegmented abdomen 

 devoid of appendages. 



Ostracoda. The body is entirely enveloped in a bivalve shell, into which also 

 the limbs can be withdrawn. The whole body is unsegmented. Besides the 5 pairs 

 of cephalic limbs there are only 2 other pairs, so that the trunk appears extraordin- 

 arily reduced. 



Copepoda. A shield or shell-fold is here in all cases wanting. The manner of 

 life of the animal leads to very various modifications of the body. In most free- 

 living gnathostomatous Copepoda the trunk is distinctly segmented (Fig. 194). We 

 can distinguish in it a thorax of 5 segments carrying appendages, and an abdominal 

 region also of 5 segments, but without appendages. The most anterior thoracic 

 segment is fused with the head to form an incomplete cephalo-thorax. The abdomen 

 ends in 2 spine- or bristle-carrying processes which diverge in a fork (furcal pro- 

 cesses). In certain female Noioddphydte each of the 4 free limb-carrying segments 

 has on the dorsal side an unpaired wing-like fold. 



In the siphonostomatous Copepoda and Argulidm we observe an advancing oblitera- 

 tion of the segmentation of the trunk and a reduction of the abdomen as the parasitic 

 manner of life becomes more marked. The body, in a few of the parasitic Siphon- 

 ostomata (Lernccidce, Lcrnmopodidce, Chondracanthidce), assumes such various and 

 extraordinary forms that no similarity to other Crustaceans can be recognised. 



Cirripedia. These, in an adult condition, are attached or parasitic animals. 



