FISTULIDA ; HOLOTHURIA ; SIPUNCULUS. 463 



1021. Although the body is not furnished with any complete 

 calcareous skeleton, we occasionally find a circle of plates around 

 the mouth. These afford attachment to the muscles, which 

 stretch along the entire surface of the body, and which are con- 

 cerned in its alterations of form. The mouth does not lead to a 

 distinct stomach ; but it is the commencement of a long alimentary 

 tube, of nearly uniform diameter throughout ; which, after many 

 convolutions, terminates at or near the opposite extremity of the 

 body. The Holothuria has the power of drawing inwards its 

 tentacula, and of closing its mouth around them ; so that no 

 appearance of them can be seen. It improbably by a movement 

 of this kind that it swallows its food, which seems to consist of 

 almost any kind of organised matter that falls in the way of the 

 animal ; for the intestine is generally found distended with sand, 

 in which may be detected the remains of corals, sea-weeds, and 

 other marine substances. There are some interesting points in 

 the internal organisation of this group, which tend to show its 

 relationship to the Articulata ; and their analogy to that group 

 is further shown in their power of comparatively active locomo- 

 tion. By the flexibility of their integument, and the muscular 

 fibres with which it is furnished, they are able to swim and crawl 

 with facility. Some of them frequent deep waters ; whilst others 

 are found among rocks and floating sea- weeds, at no great distance 

 from the shore. 



1022. Near the Holothuria, we are probably to place the 

 Sipunculus and its allies ; although the worm-like aspect of 



their bodies, and the entire ab- 

 sence of either prickles or tubular 

 feet from the skin, would lead us 

 to suppose them entirely different 



FIG. 609.-SIPUNCULUS. in character. In fact they consti- 



tute an extremely aberrant group 



just such as the philosophic Naturalist might expect to find 

 between classes formed upon types so different as the Radiated 

 and Articulated. The mouth is not surrounded by radiating 

 arms, as in the Holothuria, but it is armed, in many species, 

 with a set of teeth arranged in a pentagonal form, like those of 



n2 



