232 THE CRUSTACEA 



in this sub-order, however, is not unfrequently reduced to five, 

 four, or even three pairs. In Hyperiidea the last pair of thoracic 

 legs never carry branchiae and the number may vary from five 

 to two pairs. In Caprellidea as a rule only two pairs of branchiae 

 are present, on the fourth and fifth thoracic somites, but in some 

 genera of Caprellidae there is an additional pair on the third somite 

 (that of the second gnathopods). 



The oostegites spring from the inner surface of the coxopodites 

 on the proximal side of the branchial plates. Four pairs are 

 commonly present, on the third to the sixth thoracic somites, but 

 the number may be reduced. In the Caprellidea (Fig. 134, in} 

 two pairs only exist, on the fourth and fifth somites, but in 

 addition a pair of small valvular appendages covering the external 

 apertures of the oviducts on the sixth somite are probably to be 

 regarded as vestiges of a third pair. Vestigial oostegites are 

 stated to occur normally in the male sex in a few cases (e.g. 

 Cycimus globicipitis and Synurella polonica). 



It is very characteristic of the Amphipoda that the abdominal 

 appendages are sharply divided into two groups differing in 

 structure. In the majority of Gammaridea (Fig. 132) and 

 Hyperiidea (Fig. 136) all the six pairs are present, the first three 

 pairs are turned forwards and consist of a peduncle carrying two 

 subequal rami, each of which is multiarticulate and fringed with 

 long setae ; the inner side of the peduncle bears distally a number 

 of retinacula, and the first joint of the endopodite has internally 

 one or two peculiar "cleft spines" which no doubt serve the same 

 purpose of coupling together the pair of appendages. These limbs 

 are the chief natatory organs among Amphipoda, and they also 

 serve when the animal is at rest to maintain a current of water 

 over the gills. The last three pairs of abdominal appendages are 

 directed backwards and are generally similar to each other, so that 

 the name uropods is commonly used to include them all, though 

 in not a few cases the last pair retain, in details of form and 

 size, some mark of that differentiation from the preceding append- 

 ages which they show in other orders of the Malacostraca. As a 

 rule, the three pairs are biramous ; not unfrequently the exopodite 

 of the last pair consists of two segments, but except for this the 

 rami of all are unjointed. Sexual differences not unfrequently 

 occur in the size and structure of the last pair, but in no 

 Amphipoda are any of the pleopods modified as copulatory organs. 

 The abdomen of most Caprellidea is unsegmented and may bear 

 vestigial appendages to the number of three pairs (Fig. 134, aid). 

 An interesting link with the normal Amphipoda is constituted by 

 the genus Cercops, where five distinct somites and a terminal piece 

 (perhaps the telson) are present. The first and second somites 

 bear, in the male sex only, minute filiform appendages of two 



