202 ECHINODERMA TA. 



The adults are radially, the larvae bilaterally, symmetrical. 



Lime is deposited in the mesodermic portion of the skin, 

 and often of other parts. 



A peculiar system, known as the water- vascular, attains 

 great development, and has respiratory or locomotor functions. 

 It is not unlikely that this peculiar system originally had 

 an excretory significance, nor does it seem to have entirely 

 lost this function. The main branches of this system, and 

 the nerves and blood-vessels as well, exhibit in the majority 

 a iive-rayed stellate arrangement. 



There is a remarkable metamorphosis in development. 



Most Echinoderms have the power of casting off and 

 regenerating parts of their body. 



The average habit is sluggish. 



General Survey. — Hseckel compared an ordinary five-armed 

 starfish to a colony of five worms joined by their heads. 

 Each arm is anatomically complete in itself, with ventral 

 nerve, terminal eye, blood-vessel, water-vessel, digestive 

 caeca, and reproductive organs. There is also a certain 

 independence of life, for an arm cut off can grow the other 

 four. The peculiar life-history "was regarded by H^ckel 

 as an illustration of alternation of generations. A larva, 

 equivalent to one worm, gave origin by budding to a second 

 stage equivalent to five. Beginning with an ancestral star- 

 fish {Pentastrcea), Hsckel interpreted the brittle-stars and 

 feather-stars as modifications of this, while he believed 

 that in another direction centralisation had resulted • in 

 the sea-urchins, and in the yet more aberrant sea-cucumbers. 



Now, however, the prevalent view is very different. The 

 embryologists do not accept the "five worm theory," nor 

 do they regard the metamorphosis as an alternation of 

 generations. The Holothurians are supposed to be, in form 

 at least, nearest to the worm-like ancestor, and the inter- 

 pretation which regards the five-armed starfish as a decen- 

 tralisation of a flattened sea-urchin is more plausible than 

 that which regards the sea-urchin as the concentration of a 

 puffed-up starfish. 



But without discussing speculations in regard to the 

 hypothetical worm -like ancestors of all Echinoderms, without 

 departing from forms which we know, we may state the 

 following generally accepted conclusions : — (a) The Holo- 



