IN THE ANIMAL KINGDOM. 88 



the ova and spermatozoa are borne directly by the polypi- 

 form zoophytes into which the germ passes, by a continuous 

 course of development,* but in the compound polypes allied 

 to Coryne and Sertularia, and in the Calycophoridse and the 

 Physophoridae of Huxley — all referable to his lower section 

 of Hydrozoa — we have, as a very common arrangement, an 

 alternation of forms, occurring in what has been distin- 

 guished as the Gamomorphic stage, i.e., in connection with 

 the evolution of the proper reproductive organs — the sexual 

 elements being developed, not in organic union with what 

 may be considered as the typical form, but in peculiar zooids 

 detached from it. These go under the name of Medusoids, 

 and though there are great diversities of detail, which \\'\\\ 

 afterwards be adverted to, in their mode of origin from the 

 polypiform zoophytes, and in their size and completeness of 

 organization, there is a certain uniformity in their general 

 structure, which consists of a natatory organ, developed 

 round the spermatic or ovarian sac, and assuming the cam- 

 panulate form, so prone to repeat itself at all points in this 

 type of organization. 



The sexual zooids of the compound polypes are of very 

 minute size and rudimentary organization, while those of 

 the Lucernarian section acquire a much larger size and more 

 elaborate structure ; so that although the two forms were 

 associated in one order — as the naked and hood-eyed Me- 

 dusae — long before their true derivation was known, yet the 

 idea of their being both alike the homologues of detached 

 ovaries was long of making its way to the general accept- 

 ance it has now obtained, on account of the chsparity be- 

 tween the structures themselves, and still more on account 

 of the disproportion of their development to that of their 



* The reproductive process seems to be direct also in the Ctenophora, 

 from tlie few observations we have on the subject. See a notice of the 

 development of Cydippe, in Edinb. New Philos. Journal, N.S., IV., p. 89, 

 by Dr. T, S. Wright. 



