S4 SURVEY OF THE REPRODUCTIVE PROCESS 



respective parent stocks ; for wliile in the Hydroid and 

 Sertularian Polypifera the zoophytic stem is a far more con- 

 spicuous object than the Medusoid, the polypoid stock, or 

 larva, as it has been incorrectly termed, of the other form 

 is quite . an insignificant organism, in comparison of the 

 colossal hood-eyed Medusae which originate from it. 



The Medusa-zooids seem to be almost universally of 

 separate sexes, and most commonly it would appear that all 

 those from one polype or polypidom are of the same sex — 

 an arrangement comparable to that termed dioecious in 

 botany.* 



As the reproduction of the Hydrozoa will again come 

 under review, in considering the nature and relations of the 

 phenomena of alternation, it seems unnecessary at present 

 to go into farther details on the subject. 



§ 4. ECHINODERMATA. 



This division of animals, so natural in itself, but so 

 puzzling in its relations to other leading gToups, presents 

 us with phenomena in the protomorphic stage of develop- 

 ment, not inferior in interest to the gamomorphic zooids of 

 the class last under consideration. From the obsei-vations 

 of various naturalists, and particularly of Prof J. MUller, 

 who has been the principal labourer in this field, it appears 

 that the germ of an Echinoderm has at first the character 

 of a ciliated Infusorian, with a tendency to a bilateral form. 

 In some species of Stamsh (Eckinaster, Asteracanthion) the 

 development of the typical Echinoderm takes place by a 

 nearly direct process. The ciliated body soon manifests a 

 distinction of parts, a four-lobed portion appearing at one 

 extremity, by which the nascent animal adlieres to any sup- 

 port ; but this appendage is more than a mere pedicle of 



* Dr. Wright in Edin. Philos. Journal, N.S., IV., 88. 



