358 THE INVERTEBRATA 



the genital openings, is fused ventrally with the first abdominal. In 

 the male, it bears a pair of ventrolateral processes in each of which is 

 the terminal portion of a vas deferens, with a protrusible penis which 

 probably represents an appendage. In the female there is here a 

 median, ventral, projecting egg pouch, which, like the penes, is held 

 to represent a pair of limbs. The abdomen consists of seven simple, 

 limbless somites and a telson which bears a pair of caudal rami as 

 narrow, pointed plates, fringed with bristles. 



The alimentary canal begins with a short, vertical fore gut, or 

 oesophagus. This leads to a mid gut which continues as far as the 

 telson, where it is succeeded by the hind gut or rectum. The mid gut 

 is somewhat wider in the head, where it is known as the stomachy than 

 in the trunk, where it is called the intestine. From the stomach pro- 

 ceeds a pair of sacculated diverticula ("liver"). The /oo^ consists 

 partly of coarse detritus gathered by the trunk limbs from the bottom 

 of the pool, and partly of small organic particles, especially unicellular 

 algae, which are strained off from the water by the trunk limbs in the 

 following manner (Figs. 239, 240). The space which exists between 

 each limb and that behind it is enlarged at the forward stroke, which 

 finishes with the limbs vertical, and narrowed at the back stroke, which 

 ends with them roughly horizontal, lying against the body. During 

 the forward stroke the enlarging of this space exerts a suction. The 

 proepipodites, exopodite, and large distal endite are drawn back by 

 the suction and pressed back by the resistance of the water, till they 

 reach the limb behind and so convert the space just mentioned into 

 a chamber which is closed except on the median side, where it is 

 separated only by the backwardly directed bristle fringe from the 

 median gully between the limbs of the right and left sides. From this 

 gully, therefore, water is drawn into the chambers at the sides as they 

 enlarge, particles which it contains being strained off by the bristles 

 and remaining in the gully. The latter is of course replenished by the 

 entrance of water from the ventral side. During the back stroke, the 

 chambers, as they become smaller and the pressure of the water in 

 them rises, open owing to this pressure lifting the structures which 

 had closed them ; and the water they contain is driven out and back- 

 ward in two ventrolateral streams, the animal being driven forwards. 

 Thus the same movement of the limbs serves both for the gathering 

 of food and for swimming. The particles which are retained in the 

 median gully are drawn dorsalwards because the suction of the side 

 chambers is greatest where they enlarge most, at the bases of the 

 limbs, and so get into a median food groove of the ventral surface. 

 There they are carried forward to the mouth by a minor stream, which 

 is said to be caused by the escape forwards at the bases of the limbs 

 of some of the water contained in the lateral chambers at a certain 



