SIPUNCTJLTIS. 



211 



Fig. 105. 



a circumstance that distinguishes them remarkably from the Echino- 

 derms we are now considering ; for in Sipunculus we find a digestive 

 canal six or seven times the length of the animal, within which it is 

 folded upon itself in various distinct convolutions. Leaving the stomach, 

 if we may so call the dilatation above alluded to, it passes down (d d d) 

 nearly to the tail, where it is reflected upon itself, and mounts up again 

 as far as the point where it com- 

 menced ; here it again turns back, and, 

 once more reaching the bottom of the 

 tegumentary sac, becomes a second 

 time directed upwards, and re-ascends 

 as far as the point e, where the anus 

 is situated. 



(549.) It is easy to account for this 

 extreme length of the intestine when 

 we consider the nature of the mate- 

 rials used as food, and the small pro- 

 portion of nutriment contained among 

 the sand and broken shells found in 

 the digestive canal : but the remark- 

 able position of the anal aperture is 

 only explicable by a reference to the 

 peculiar habits of the creature; for 

 (living, as it does, in a narrow exca- 

 vation bored in the sand, from which 

 it seldom issues), had the excrements 

 been discharged, as in Holothuria, 

 through a terminal orifice, their con- 

 stant accumulation at the bottom 

 would soon expel the animal from its 

 retreat ; but, by the arrangement 

 adopted, it is only necessary that the 

 anterior part of the body should be 

 protruded from its concealment, and 

 the excrementitious matter may be 

 cast out without inconvenience. The 

 intestine is retained in situ and sup- 

 ported at all points by innumerable 

 tendinous bands, that arise from the 

 interior of the muscular walls of the 

 body and form a kind of mesentery. 



(550.) In Sipunculus, the character 

 of the circulating system is in all 



essential points strictly analogous to that of the other Echinodermata ; 

 and moreover, from the superior concentration visible in every part, we 



Anatomy of Sipunculus : a, oral tentacles ; 

 b, cesophagus ; c, stomach ; d d d, intestinal 

 canal ; e, position of anal orifice ; f,p, ovaria ; 

 g, external orifice of ovary; h, ampulla Poli- 

 ana ; i, cerebral ganglia ; I, heart ; in, intes- 

 tinal vein; n, branchial vessel; o, aortic 

 trunk. 



