ECHINODERMATA. 



181 



mounts up again as far as the point where it commenced ; here it 

 again turns back, and, once more reaching the bottom of the tegu- 

 mentary sac, becomes a second 

 time directed upwards, and re- 

 ascends as far as the point (e), 

 where the anus is situated. 



It is easy to account for this 

 extreme . length of the intestine 

 when we consider the nature of 

 the materials used as food, and 

 the small proportion of nutri- 

 ment contained among the sand 

 and broken shells which fill the 

 digestive canal : but the re- 

 markable 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 excavation bored in 

 the sand, from which it seldom 

 issues, had the excrements been 

 discharged, as in Holothuria, 

 through a terminal orifice, their 

 accumulation at the bottom of 

 the hole 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 pro- 

 truded from its concealment, 

 and the excrementitious matter 

 may be cast out without incon- 

 venience. The intestine is retained in situ, and supported at 

 all points, by innumerable tendinous bands, which arise from the 

 interior of the muscular walls of the body, and form a kind of 

 mesentery. 



(223.) In Sipomulus, the character of the circulating system is in 

 all essential points strictly analogous to that of the other Echinoder- 

 mata; and moreover, from the superior concentration visible in 

 every part, we have the multiplied organs of the other families ex- 

 hibiting so much simplicity of arrangement, that, whatever may 



