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



related to each other, and have no hesitation in placing them to- 

 gether in a single group, or phylum, known technically as the 

 Echinodermata. 



These Echinoderms may be distinguished from other animals 

 by certain well marked characters. As the name implies, the 

 skin in most forms is provided with calcareous spines which pro- 

 ject out from all sides of the body. Such spines are wanting 

 however in the sea-cucumbers. Other calcareous plates are found 

 in the skin. In most forms such plates unite to make a more or 

 less complete shell which serves to protect the delicate internal 

 organs. Most of them have likewise a distinct radial symmetry, 

 in that similar parts are repeated around the central axis, as 

 illustrated by the arms of the starfish or serpent star. With few 

 exceptions they have a well developed water-vascular system, 

 consisting of a circular canal around the mouth, with radial 

 branches leading out to the periphery of the body. From the 

 radial canals numerous branches lead out into the skin in certain 

 regions of the body, and grow out into delicate tubular or finger- 

 like projections. 



These delicate projections are variously modified in different 

 parts of the body for the performance of certain special functions. 

 In all cases they serve as sense organs and as respiratory organs. 

 If they have no further function they are known as tentacles; 

 but in many cases they serve also in the locomotion of the body, 

 and are variously known as tube-feet, sucker-feet, or pedicels. 



These animals are found only in salt water, with the exception 

 of one or more tropical species which are able to live in brackish 

 water. 



