320 THE INVERTEBRATA 



typical features of such limbs but are remarkable for the distal 

 position of the exopodite and for the very long basal endite, which 

 probably represents two, of which one may be the gnathobase (p. 306), 

 unless the latter has been lost altogether. The fringe of long bristles 

 on the median border is, in life, directed backwards, roughly at right 

 angles to the main plane of the limb. The twelfth thoracic somite, 

 upon which are 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 pro- 

 trusible 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 stomach, than 

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

 a pair of sacculated diverticula (''liver"). The food consists of small 

 organic particles, especially unicellular algae, which are strained off 

 from the water by the trunk limbs in the following manner (Figs. 228, 

 229). 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 proepipodites, exopodite, and large distal endite are 

 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 be- 

 tween the limbs of the right and left sides. From this gully, there- 

 fore, 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 

 backward 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 fall (the animal being on its back), 

 dorsal wards into a median food groove of the ventral surface. There 



