CIRCULATORY SYSTEM. 87 



The rectum, lined by membrane continuous with that 

 investing the thorax (and seen through it, in PI. 26, fig. 8, c), 

 extends inwards to about opposite the bases of the third or 

 fourth pairs of cirri. It is longitudinally plaited ; the ends 

 of the folds forming a sort of valve where joined on to the 

 stomach. It is coated by circular, transverse muscular 

 fibres : judging from the movements, the anus itself is 

 surrounded by a strong sphincter muscle. The anus opens 

 on the dorsal surface of the thorax (fig. 8, b) ; but as in the 

 genera, in which caudal appendages occur, it opens under 

 them, the orifice, I believe, is homologically terminal, and 

 owes its dorsal aspect to the aborted state of the whole 

 abdomen, and to the great development of the probosci- 

 formed penis ; for the anus may be said to be situated on 

 the dorsal base of this organ. 



Altogether we see that the alimentary canal is of a very 

 simple structure. The food, judging from the contents of 

 the stomach, seems generally to be composed of infusoria 

 and minute animals : but in the case of Tetraclita, I have 

 been surprised at the size and number of the included 

 amphipod, isopod, and entomostracan Crustaceans, in one 

 case, together with an annelid. I have, also, sometimes 

 seen some confervoid matter within the stomach. 



Circulatory System. — On this subject I can add nothing, 

 except to express my conviction that there is no heart, or true 

 vessels; the circulation being strictly lacunal. A passage has 

 often been quoted from Poli, in which he states that he saw a 

 pulsating organ, close above the anus ; but I have seen this 

 movement, which appeared to me to be a convulsive twitch- 

 ing of the sphincter muscle of this orifice. The largest 

 lacunal channel extends down the middle of the rostral 

 compartment of the shell : and this answers to the rostral 

 channel down the peduncle in the Lepadidse. Large 

 nerves and the main pair of unbranched ovarian tubes 

 (PI. 25, fig. 1, leading into g) extend along this channel. 

 At the basis (at least in Coronula) this channel joins on to 

 a large circular one, running all round the sack, and send- 

 ing off branches into the mass of ovarian tubes and caeca. 



