THE AMPHIPODA 233 



segments which appear to correspond to the first and second 

 pleopods of the normal type, while the fourth and fifth somites 

 carry in l»oth sexes stout two-segmented limbs answering to the 

 first and second uropods. In most other Capiellidae the rudimentary 

 appendages are more fully developed in the male than in the female 

 sex, but in a few the abdomen is quite without appendages. In 

 the male Cyamidae a median appendage is present which seems to 

 result from the fusion of a jiair of uropods. In JiKjolpeUa the first 

 three pairs of abdominal limbs are represented by small triangular 

 plates, sometimes with a minute basal segment, Avithout any trace 

 of rami. The last pair is vestigial. 



Alimentarfi Si/dem. — The alimentary canal of the Amphipoda 

 appears to differ from that of most Isopoda in the much gieater 

 development of the mid-gut region, which forms the greater part of 

 its length. The stomodaeum forms a masticatory or triturating 

 stomach, the structure of which appears to be fairly uniform 

 throughout the Gammaridea and Caprellidea, and to be more or 

 less simplified in the Hyperiidea. When fully developed it presents 

 anteriorly two lateral ridges projecting into the cavity, armed with 

 spines and stiff setae. These ridges are moved by powerful muscles 

 passing outwards to the body-wall on either side, and appear to be 

 the most important instruments of trituration. Posterior to these 

 are two pairs of setose ridges running more or less transversely, 

 while in the floor of the cavity is a strong ridge ending behind in a 

 free tongue-like process and carrying anteriorly four comb-like 

 rows of iridescent setae. The chitinous lining of the stomodaeum 

 projects backwards into the beginning of the mid-gut as a cuticvdar 

 funnel. In Fhroniina, where the apparatus appears to be adapted 

 for straining rather than for masticating the food, the whole stomach 

 is telescoped for a little way into the capacious mid -gut. The 

 hepatic caeca are generally four in number and of considerable 

 length, but in a few genera of Gammaridea and in most Hyperiidea 

 only one pair is present, while in the Caprellidea the ventral pair 

 and in Phronima both pairs remain rudimentary. Just above the 

 point where the hepatic caeca communicate with the gut by a 

 common aperture on each side, a short dorsal, forwardl}^ directed 

 caecum, unpaired in Gammaridea, paired in Hyperiidea and 

 Caprellidea, arises from the anterior end of the mid-gut. At the 

 posterior end of the mid-gut, at its junction with the proctodaeum, 

 a second pair of caeca of very varying size is commonly ])resent in 

 the Gammaridea. In a few cases only a single unpaired caecum is 

 present {Melitu), and in Si/mirclhi, though two tubes are present in 

 the young, that on the right side atrophies and only that on the 

 left persists. Similar paired caeca are only exceptionally present 

 in Hyperiidea {I'ibilia) and Caprellidea {CnpreUa). To these 

 l)OSterior caeca of the mid-gut an excretory function has been 



