330 THE INVERTEBRATA 



very short-bodied and completely enclosed in a bivalve shell formed 

 by the carapace. Whereas, however, in the Cladocera it is by trunk 

 limbs that food is gathered, in the Ostracoda that function is per- 

 formed by limbs of the head. The trunk limbs, which have lost the 

 functions of swimming and respiration as well as that of feeding, 

 serve relatively unimportant subsidiary purposes, and are reduced, at 

 most, to two pairs. Some members of the class carry shortening to 

 an extreme pitch by contriving to dispense with one or both of these 

 pairs. (3) The members of the class Copepoda (Fig. 249) also feed by 

 means of appendages on the head, though they use these differently 

 from the Ostracoda. In contrast to that group they have no carapace, 

 and they have retained a trunk of some ten somites, of which the first 

 half-dozen bear limbs which are specialized organs of swimming. The 

 hinder part of the trunk is without appendages, save a pair of styles 

 on the telson, often shows coalescence of somites, and may become 

 a mere stump. Some of those members of this class which are para- 

 sitic lose in the adult female the segmentation and most, or even all, 

 of the appendages. (4) In the small class of parasites known as 

 Branchiura (Fig. 254), which are sometimes placed in the Copepoda, 

 but differ from that group in possessing compound eyes and in other 

 important respects, there are carapace-like lobes at the sides of the 

 head, but these do not enclose the trunk, and the general build of 

 the body and the form and function of the thoracic limbs simulate 

 those of a copepod. The abdomen is much reduced. (5) The class 

 Cirripedia or barnacles, which as larvae attach themselves by their 

 antennules to some object upon which they henceforward lead a 

 sedentary life under the protection of a large, mantle-like carapace, 

 bear, upon the same trunk somites as do the Copepoda, limbs which, 

 like those of the latter group, are biramous. These appendages, how- 

 ever, are used , not for swimming, but for gathering food-particles from 

 the water ; while of the head appendages the antennae are absent and 

 the others are much reduced and not used in gathering food. The least 

 specialized members of this class are, in respect of segmentation and 

 appendages, on a par with the best-segmented of the Copepoda. Most 

 cirripedes, however, (the ordinary barnacles, Fig. 255) have lost the 

 whole of the hinder (abdominal) region of the trunk. Others are 

 deficient in the appendages of further somites, and the series ends 

 with the sac-like parasites of the order Rhizocephala (Fig. 260). 

 (6) The class Malacostraca (the highest crustaceans, including various 

 "shrimps", slaters, sandhoppers, crayfishes, etc.) obtain their food 

 with the limbs on the anterior region (thorax) of the trunk, and, in 

 primitive cases in which it is gathered as particles, strain it from the 

 water with the last pair of appendages of the head (the maxillae). The 

 thoracic limbs retain also the function of locomotion and normally 



