228 ARTHUR B. SHIPLEY. 



was 19'08 grms., that of the sand 10"03 grms. In two of 

 them the sand weighed more than one half the total weight, 

 the body being in one case 34*4 grms. and the sand IS'72, and 

 in the other 15 '05 grms. and 9*45. The contents of the 

 intestine consisted of blackish sand with a few Foraminifera 

 mixed with it. In spite of the considerable amount of sand 

 which these figures show to be contained in the intestine, the 

 wall of this tube in all the Sipunculids with which I am 

 acquainted is excessively thin, and apparently but poorly 

 adapted to retain the sharp and jagged pieces of sand which lie 

 within it. A similar tenuity of the wall of the alimentary 

 canal also occurs in Echinids and Holothurians. Although 

 this wall is so thin I have never found a Sipunculid with its 

 intestine ruptured, so that in spite of appearances it seems to 

 serve its purpose well. 



I have mentioned above that I am of opinion that the 

 respiration of Onchnesoma is carried on through the walls of 

 the intestine. The seat of the process of respiration is still a 

 debatable point in the anatomy of the unarmed Gephyrea. 

 Of the two recent authors who have written on the anatomy 

 of Sipunculus, Mr. Andrews^ is convinced that the tentacles 

 act as branchiae, whilst Mr. Ward^ is of opinion that they do 

 not. In Onchnesoma, at any rate, there cannot be any 

 question as to the respiratory action of the tentacles, as the 

 latter are entirely absent. In other Sipunculids the tentacles 

 may to a slight extent serve as the organs of respiration, but 

 the closed vascular system which supplies them with blood is 

 of such a very limited extent that it would only suflSce for a 

 small portion of the body ; on the other hand, it seems to me 

 quite possible that the brain, which is almost entirely sur- 

 rounded by this system, may obtain its oxygen from it. 



The chief circulating medium in the body of the unarmed 

 Gephyrea is undoubtedly the corpusculated coelomic fluid, and 



» Loc cit., p. 419. 



^ "On some Points in the Anatomy of Sipunculus nudus, L.,"HenrjB. 

 Ward, ' Bull, of the Museum of Comp. Anat., Harvard College,' vol. xxi, 

 No. 3, May, 1891. 



