ri6 CONNECTICUT GEOL. AND NAT. HIST. SURVEY. [Bull. 



of the oral and aboral ends as anterior and posterior, and the 

 terms right and left are applied as in other bilaterally symmetrical 

 animals. 



The body walls are very muscular, with enormously powerful 

 longitudinal muscles running the length of the body along the five 

 radii, which correspond to the ambulacral areas of the other 

 groups (Fig. 20). Transverse muscles serve to diminish the cir- 

 cumference of the body, and thus, by exerting pressure on the 

 fluids in the body, cause the animal to elongate. The interaction 

 of the two sets of muscles enables the creature to move in a 

 worm-like fashion. 



The general anatomical features of the holothurians may be 

 well illustrated by a study of Thyone, as described below. 



Internal Anatomy of Thyone. — Few marine invertebrates 

 are more satisfactory for the study of internal anatomy than the 

 common Thyone. For this reason the more conspicuous of its 

 organs will be briefly described in this place, although the details 

 of the water-vascular system, blood system, and nervous system, 

 the description of which would require more space than is here 

 available, must be omitted. 



So powerful are the contractions of the body walls when the 

 animals are injured, that, unless some special precautions are 

 taken in killing the specimens for dissection, the body may be 

 ruptured, and the whole crown of tentacles and a portion of the 

 viscera may be ejected. A convenient method of avoiding this 

 trouble is to seize the living holothurian immediately behind the 

 tentacles with strong forceps and plunge it into boiling water. 

 If the experiment is successful, the tentacles will remain partially 

 expanded, and the body walls intact. 



For a study of the internal organs, the body should be opened 

 with scissors along the whole length of the ventral surface. 



On opening the body, the viscera are found to be situated in a 

 large central cavity, the body cavity, or coelom. In life the 

 space between the viscera and the body walls is filled with the 

 coelomic fluid. 



Alimentary canal.— Of the ten branched tentacles which 

 surround the mouth, two are much smaller than the others, and 

 will serve to indicate the ventral side of the body (Plate XXX). 

 If a slit is made through the ventral body wall throughout the 



