146 LIFE BY THE SEASHORE. 



and by the fact that the edges of the holes of the anchor 

 plates are serrated. As is to be expected from the habitat, 

 it lives on the organic particles contained in sand, and the 

 alimentary canal with its contained sand can be seen shining 

 through the transparent body-wall. As in Cucumaria, the 

 tentacles can be completely retracted, and the animal is 

 then very worm-like in appearance. 



This concludes the consideration of our common littoral 

 Echinoderms. The forms mentioned should give the student 

 a general idea of the main points of structure, and should 

 serve to indicate the general interest of the group. Our 

 common littoral forms are adapted to very various conditions 

 of life, and while retaining certain common peculiarities of 

 structure, present in a most interesting way what are known 

 as adaptive characters. One very interesting point in regard 

 to the group is, that the development is usually very 

 indirect, the larvae being quite unlike the adult, and adapted 

 for very different conditions. The larvae of our common 

 shore species are to be sought in the tow-net near the 

 surface of the sea, and are often very quaint in form. The 

 study of the development is beyond our scope, but this 

 chapter would be incomplete if it did not mention the fact 

 that not only are larvae and adults very unlike one another, 

 but that the former are converted into the latter by a 

 remarkable process of metamorphosis. Further, on account 

 of their marine habit, and the abundance of lime contained 

 in the tissues, the Echinoderms are abundantly represented 

 as fossils, and their geological history is in consequence 

 better known than that of most animals. 



