180 ECHINODERMATA. 



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

 easily separable by maceration or immersion in spirit of wine ; 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 Siponculus saccatus. 



The muscular investment, placed beneath the skin, is composed 

 of strong fasciculi arranged in three distinct layers. The external 

 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, extending from one extremity of the body to the other. 

 Such an arrangement is evidently sufficient for the general move- 

 ments of the creature ; 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 inverted and drawn inwards. 



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 sensibility and power of con- 

 traction ; but, as we have found to be generally the case among 

 the Echinodermata, 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 

 Siponculi are nourished. 



(222.) 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 

 relationship which exists between the apodous Echinodermata 

 and the Holothuridse. The oesophagus (fig. 76, 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 pecu- 

 liarity to distinguish it from the rest of the intestine. In the 

 ANNELIDA, the digestive apparatus is invariably straight, travers- 

 ing the body from one extremity to the other, a circumstance which 

 distinguishes them remarkably from the Echinoderms we are now 

 considering ; for in Siponculus 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, J, d,) nearly to the tail, where it is reflected upon itself, and 



