226 THE CRUSTACEA 



flexed ventrally between the third and fourth somites. The large 

 coxal plates (ex) on the thoracic somites projecting downwards 

 increase the depth of the body and add to the appearance of lateral 

 compression. Even within the sub-order Gammaridea, however, 

 this typical form is sometimes departed from by attenuation of the 

 body or by its dorso-ventral flattening, which, in Pereionotus and 

 some other genera, is carried so far as to give the general appear- 

 ance of an Isopod. In the sub-orders Hyperiidea and Caprellidea 

 the range of modification is much greater. The Hyperiidea, while 

 departing the less widely, on the whole, from the typical form, 

 include such extreme types as the balloon-like Mimonectes and the 

 almost linear Rhabdosoma (Fig. 133). The Caprellidea differ from 

 both the other sub-orders in the vestigial condition of the abdomen 



FIG. 133. 

 Rhabdosoma piratiun (Hyperiidea). (After Stebbing, from Ency. Brit.) 



and the coalescence of the second thoracic somite with the head ; 

 they comprise two families of widely different facies, the filiform 

 Caprellidae (Fig. 134) and the flattened, Isopod-like whale-lice, 

 Cyamidae (Fig. 135). 



The eyes, when present, are sessile on the sides of the head. 

 Sometimes they coalesce in the median line, and in some Oedi- 

 cerotidae the fused eyes are borne on a projecting frontal lobe. 

 In Ingolfiella, distinct eye-lobes are present (although the eyes 

 are apparently deficient), defined by suture-lines from the antero- 

 lateral margins of the head -region. It is possible that these 

 lobes may represent the eye-lobes of Tanaidacea and the ocular 

 peduncles of more primitive Malacostraca, though the great 

 specialisation of Ingolfiella in other respects is rather against this 

 view. 



Appendages. The antennules (Fig. 132, a') consist typically of 

 a peduncle of three segments carrying two flagella. The outer 

 flagellum is usually long and multiarticulate, while the inner (ace) 



