294 THE INVERTEBRATA 



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 parasitic 

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

 appendages. In the Branchiura (Fig. 243), which are usually classed 

 with the Copepoda but differ from the rest of that group in possessing 

 compound eyes and in other respects, the abdomen is much reduced. 

 (4) 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 the Copepoda limbs which, like 

 those of the latter group, are biramous. These appendages, however, 

 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. 244) 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(¥ig.2\Ci). (5) 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 are adapted 

 for respiration by the presence upon them of gills, which are usually 

 protected by a carapace of moderate size. Thus this region of the 

 body of the Malacostraca is, in its own ways, as many-functioned as 

 the corresponding part of the trunk of Chirocephalus . The Malacos- 

 traca maintain in typical cases (Figs. 256, 268) the swimming function 

 of the limbs on the hinder portion (abdomen) of the trunk, and some 

 of the class have found other uses (ovigerous, copulatory, etc.) for 

 these appendages. Accordingly there is seldom any reduction in the 

 fixed number of fourteen (or fifteen) trunk somites which, arranged 

 always in a thorax of eight and an abdomen of six (or seven), cha- 

 racterizes the class. Nevertheless in a few members of the group the 

 abdomen has become a limbless and unsegmented stump, and 

 in the crabs (Fig. 270) and some others of the highest order {Deca- 

 poda) it is reduced. 



The name Entomostraca was formerly used in the classification of 



