10 



Echinoderms. 



By W. Manger. Read April itfk, 1905. 



This is a somewhat technical subject, and it is difficult to condense 

 the mass of available information into the very narrow compass of 

 these notes. I can only point out the general characteristics of a 

 group of animals whose study becomes more and more interesting 

 with increasing knowledge of the subject. 



Echinoderms are divided into seven classes, each of which is 

 again divided into orders. The name Echinoderm is derived from 

 the Greek echinos, a hedgehog, and derma, skin. The group is 

 comprised of (among others) the star-fish, the sea-urchin, the brittle 

 star, the feather star, the crinoids or stone lilies, and the sea 

 cucumber ; the first three are well known to all of us, as are also' 

 the fossil sea-urchins of the chalk which are so common on the 

 downs of England. Echinoderms are of great geological age and 

 were very abundant in the earlier periods of the world's history. 



Two groups, the Blastoids and Cystoids, have entirely disappeared, 

 and the stalked Crinoids or stone lilies are becoming very rare. 

 Though these animals differ very much in shape, a slight examina- 

 tion will discover many points in which they resemble one another 

 and differ from other creatures ; they are therefore placed in one 

 great group, the Echinodermata. 



Echinoderms are found in all seas and extend to great depths of 

 ocean. They are most abundant in the tropical seas. Most Echino- 

 derms lay their eggs in the water, where the larvae are developed 

 and swim about freely, passing through a series of remarkable 

 changes, but in a few cases the young do not pass through any 

 metamorphosis : the eggs are placed in special pouches of the body 

 of the parent in which they are hatched. Reverting to the differ- 

 ences which the animals in this group exhibit, in a star-fish, as in a 

 sea-urchin, the mouth is in the centre of the under surface and the 

 vent almost in the centre of the upper surface, with some few ex- 

 ceptions. The body of the star-fish is either pentagonal or more or 

 less star-shaped, in which case it consists of a central disc extended 

 into arms. The number of these arms varies from five (Asterias) to 

 over forty (Heliaster) ; and whereas the arms of a star-fish are simply 

 extensions of the body, containing the generative glands and pro- 

 cesses of the stomach, those of the brittle star are mere append- 

 ages to the body. The common star-fish, or five fingers, as it is 

 sometimes called (Asterias nibcns), so frequently met with on our 

 shores, has usually five arms which, as I have said, are continuous' 

 with the disc, and contain portions or prolongations of the chief 



