422 GEPHYREA chaf. 



weldonii and Ph. asser, Denchostoma signifer, S. vastus, the lumen 

 of the dorsal vessel is increased by numerous hollow blind pro- 

 cesses which it bears, hanging freely into the body -cavity. 

 Three very small genera of Sipunculids Onchnesoma, Petalo- 

 stoma, and Tylosoma are devoid of all trace of vascular system 

 and of tentacles ; the mouth opens in the centre of the anterior 

 end of the introvert. In Onclmesoma the dorsal part of the lip 

 is somewhat produced, so that the head has somewhat the shape 

 of a Doge's cap, and in Petalostoma there are two leaf-like pro- 

 cesses of the body- wall which guard the mouth. 



The extent to which the intestine is coiled varies very much 

 even in the same species ; the axis of the coil is often supported 

 by a spindle-muscle, but this is sometimes absent. The caecum, 

 which opens into the rectum of S. nudus, is again a very variable 

 structure, and when it is present varies remarkably in size. 



The food of Sipunculids seems to consist almost entirely of 

 sand, and their only nourishment must be such small microscopic 

 organisms or particles of animal and vegetable debris as are to be 

 found mixed with the sand. The alimentary canal is, as a 

 rule, quite full of sand, and yet in spite of the tenuity of its 

 walls they never seem to be ruptured. If the contents of the 

 digestive tube be washed out with a pipette, it will be found 

 that it requires considerable force to dislodge many of the 

 sand-particles lying next the wall. These are more or less 

 embedded in crypts or pockets of the wall, and as the sand 

 passes along the intestine they probably serve as more or less 

 fixed hard points, against which the sharp edges of the sand 

 particles are worn off. Amongst the sand are usually to be 

 found pieces of shell, sometimes with a diameter equal to that 

 of the alimentary canal; these are usually rounded, but their 

 angles may have been removed by attrition before they entered 

 the mouth of the Sipunculid. 



In S. tesselatus the sand is to some extent held together by a 

 mucous deposit ; in those cases where there is no sand in the 

 intestine, there is always a coagulum of mucus, and the walls 

 are contracted and thick ; when full of sand the walls are 

 tensely stretched and very thin. This thinness of the wall of 

 the alimentary canal seems ill-adapted to a diet of sand, never- 

 theless it is also met with in other great sand-eating groups of 

 animals, such as the Echinids and the Holothurians. 



