THE AMPHIPODA 231 



is reversed. 1 The distinctness of these two groups is further 

 expressed by the relations of the coxal plates. Where these are 

 small or of moderate size they are of similar form throughout the 

 series, but when, as in many Gammaridea (Fig. 132), enlargement 

 takes place, it is mainly the first* four that become expanded so 

 as to cover from the outside the basipodites, or even the whole 

 limb ; the last three coxal plates in this case generally remain 

 small and their place in protecting the gills within is taken by 

 the expanded basipodites. The two pairs of legs following the 

 gnathopods (the fourth and fifth of the thoracic series) are, among 

 the Gammaridea (Fig. 132, prp', piy>"), not unfrequently more or 

 less different from the succeeding pairs. In the Caprellidea, these 

 two pairs are vestigial or absent except in the genus Phtisica (Proto) 

 (Fig. 134, A) and its immediate allies. 



In the gnathopods of the Gammaridea every gradation may be 

 traced from the simple, non-prehensile leg to the well-formed, 

 sub-chelate, or perfectly chelate type, and even to more complex 

 forms, as in Leuc.othoe and Aora. The gnathopods of Ingoljiclla 

 have a very unusual structure, the propodite and dactylopodite 

 together forming the movable finger which is opposed to the 

 expanded carpopodite. Some of the peraeopods may show modifi- 

 cation for prehensile purposes in Gammaridea, and in Polycheria 

 all of them are sub-chelate. In the Hyperiidea much greater 

 variety occurs, and any of the peraeopods except the last pair may 

 be transformed into a chela, sometimes of large size (Fig. 136). 

 The gnathopods, in this group, are always small. In the Caprellidae 

 (Fig. 134) the gnathopods are sub-chelate and the last three pairs 

 of peraeopods are also fitted for grasping. A point of interest 

 with regard to these peraeopods is the existence in the basipodite 

 of a definite "fracture-plane" at which the limb breaks off in 

 antotomy. A similar fracture-plane is found in the legs of many 

 Decapoda, where the habit of autotomy is highly developed. 



The branchiae- (Fig*. 132, 134, 135, br Fig. 136, K) are 

 attached to the inner surfaces of the coxopodites, near the 

 posterior border. They are generally vesicular or lamellar in 

 form, and in some Gammaridea the respiratory surface is in- 

 creased by numerous transverse ridges or folds. In some species 

 of Cyamidae (Cyamus physeteiis) the branchiae are ramified. 

 Accessory branchiae occur in certain Gammaridae and Talitridae 

 on some of the thoracic limbs and also on the first abdominal 

 somite. The greatest number of branchiae is six pairs, occurring 

 on the last six thoracic limbs in many Gammaridea. The number 



1 The correlation l>etween the lateral compression of the body and this grouping 

 of the legs is well illustrated by comparison with Phreatoiciis, the only Isopod where 

 the body is laterally compressed and where the legs are divided into two groups 

 exactly as in Amphipods. 



