Glass 1. Crustacea. 



189 



they also often possess a small end-piece^ a palp. The rest of the 

 body bears a varying number of limbs, arising on the ventral 

 side, one pair to each segment. The terminal segment is frequently 

 apodous, so also may be some of the others. In rare instances all 

 these Appendages are almost or quite identical, but usually those of 

 the different segments are more or less dissimilar. Frequently, for 

 instance, the foremost are nutritive in function, and are correspondingly 

 modified, and are then called maxillipeds; the hindmost may be 

 swimming organs, whilst others, again, are ambulatory. The limbs 

 are, in short, highly specialised in form and function. 



It is, however, possible to reduce all the limbs to a common 

 type, not only those which belong to the trunk, but also those of the 

 head, i.e., the second antennae,* and the three pairs of jaws. A typical 

 crustacean limb consists of 



the following parts : (1) a ^ ^ 



main stem or e n d o p o d, 

 composed of a number of 

 joints, and constituting the 

 chief part of the limb ; (2) 

 an outer branch or e x o - 

 pod springing from the 

 second jointof the endopod j 

 unjointed, or at least not 

 divided into special pieces 

 m.oving upon one another ; 

 usually flat, and provided 

 with marginal setse ; (3) an 

 e p i p o d arising from the 

 basal joint of the endo- 

 pod, always unsegmented, 

 usually sparsely setose, thin 



skinned, and as a rule subserving respiration. Bxopod or epipod,t 

 or both, may be absent, so that the limb consists of the endopod only; 

 and even this may be more or less degenerate. On the other hand 

 certain joints of the endopod may be specially well-developed, as, 

 for instance, the basal joint of the mandible. f The exopod may be 

 flat or round, curled or uncurled, etc. 



Amiong other cephalic structures must be noticed the carapace 

 (or shell), a backwardly-directed, mantle-like fold, arising from the 



Pig. 150. Example of typical crustacean 

 limbs. A thoracic limb of Nebalia. B last maxil- 

 liped of a larval Prawn. 1 — 7 joints of the stalk. 

 ex exopod, ep epipod. Enlarged. — Orig. 



* The first antenna, though often called limbs, do not agree with the rest of the 

 appendages, but exhibit peculiar relations (the exopod is always absent from the 

 second joint, etc.) ; like the eye-stalks they are to be regarded as special appendages, 

 -which function as the supports of special sense organs (olfactory and auditory). 



t The epipod is always absent from the appendages of the head. 



t The true functional mandible is merely the basal joint developed as a cutting or 

 grinding organ ; its jointed appendage, the palp, represents the rest of the endopod. 



