210 EOIIINODERMATA. 



body, although the radiating tentacula around the mouth are evidently 

 analogous to those of the Holothuria, already described. 



(543.) The Sipunculus inhabits shallow seas, concealing itself at the 

 bottom in holes that it excavates in the sand. Having once located 

 itself, it is seldom found to quit its concealment, but, retaining its hold 

 upon the sides of the retreat which it inhabits, by dilating the posterior 

 part of its body it occasionally protrudes its head from the orifice, either 

 for the purpose of procuring food, or of respiring more freely the water 

 of the ocean. 



(544.) These animals are much sought after by fishermen, who em- 

 ploy them as baits for their hooks ; and one species, Sipunculus edulis, 

 is used in China as an article of food. 



(545.) The body is covered externally with a delicate cuticle, easily 

 separable by maceration or simple immersion in spirit; and when 

 thus detached it forms so loose a covering, that Linnseus, deceived by 

 the appearance of an animal thus preserved, applied to it the name of 

 Sipunculus saccatus. 



(546.) The muscular investment, placed beneath the skin, is com- 

 posed of strong fasciculi arranged in three distinct layers. The exter- 

 nal stratum is disposed in circular rings, beneath which spiral fibres 

 may be observed crossing each other at various angles ; and lastly, the 

 inner coat is made up of about thirty powerful longitudinal bands, ex- 

 tending from one extremity of the body to the other. Such an arrange- 

 ment is evidently sufficient for the general movements of progression ; 

 but in order to facilitate the retraction of the tentacular apparatus 

 around the mouth, eight additional muscles surround the oesophagus ; 

 and by their action the whole of the oral apparatus is completely in- 

 verted and drawn inwards. 



(547.) The tentacula around the oral orifice are the principal agents 

 employed in seizing and swallowing food, an office to which they are 

 peculiarly adapted by their great sensitiveness and power of contraction ; 

 but, as we have found to be generally the case among the Echinoder- 

 mata, sand and fragments of shell form the great bulk of the contents 

 of the intestine, so that it is by no means easy to state precisely the 

 nature of the food upon which the Sipunculi are nourished. 



(548.) The structure of the alimentary canal and of the nutrient 

 apparatus conforms too accurately with what we have already seen in 

 Holothuria to permit of a moment's hesitation concerning the relation- 

 ship that exists between the apodous Echinodermata and the Holo- 

 thurida3. The oesophagus (fig. 105, b) is narrow, and soon dilates into 

 a kind of stomachal receptacle (c) ; but, although the diameter of the 

 intestinal tube is at this point perceptibly larger than in any other 

 part of its course, there is no other peculiarity to distinguish it from 

 the rest of the intestine. In the Annelida, the digestive apparatus is 

 invariably straight, traversing the body from one extremity to the other, 



