GEOGRAPHIC DISTRIBUTION 



51 



lengthen the sea-urchin in the direction of the mouth to aboral 

 surface, and you have the form of the sea-cucumber. The 

 crinoid also reveals this plan, not so clearly defined, but it is to 

 be seen by the careful observer. The number of rays varies in 

 the starfish, the author having found them with four, six, or 

 even as many as twenty-two rays. 



Fig. 40. 1 and 2, Amphipholis squa?nata (adult), aboral and oral views. 

 3 and 4, Asterias vulgaris (small specimen), aboral and oral views. (Bul- 

 letin, U. S. F. C., 1902.) 



The Skeleton or " Test." The body wall is composed of a 

 thick leathery substance. In the mesoderm, under the epi- 

 thelium, calcareous plates arise, many of which are armed with 

 spines for protection. They are greatly protected also by their 

 resemblance to their environment. 



Geographic Distribution. All echinoderms are marine, being 

 abundant even in the deep sea. They are found in all parts of 

 the globe, but are most abundant in the tropics. At the breed- 

 ing season most of the free species frequent the shallow waters 



