164 MODE OF GROWTH OF THE RADIATES. 



In all the marine Hydroids, which are more complex in their 

 individualism than Hydra, the sexes being separate, the eggs and 

 spermatic particles are thought by Allman to be developed from 

 the endoderm. But E. Van Beneden has on the other hand shown 

 that the eggs in Hydractinia are exclusively developed from the 

 endoderm, while the spermatic cells arise from the ectoderm. 



The simplest form next to Hydra is Hydractinia, in which the 

 individual is differentiated into three sets of zooids ; i.e., a, hydra- 

 like, sterile or nutritive zooids ; b and c, the reproductive zooids, 

 one male and the other female, both being much alike externally, 

 having below the short rudimentary tentacles several spherical 

 sacs, which produce either male or female medusae. These 

 medusa buds or closed generative sacs are fundamentally like 

 the free medusse in structure. The marine Hydroids, then, are 

 universally dioecious,, and usually each colony is either male or 



A pather more complicated form is the common Coryne mirabUis. 

 Fig. 51 shows, the hydrarium with its long tentacles («) and a the 

 medusa buds, Fig. 52 its free medusa. Tubularia is a higher form, 

 and allied to the latter is still another form, Corymorpha pendula 

 (Fig. 53. After Agassiz). 



Figs. 54-58 (after A. Agassiz) represent quite fully the life his- 

 tory of another Tubularian, Bougainvillia superciliaris. Fig. 54 

 represents the hydrarium, with the sterile zooids provided with long 

 tentacles, and the medusa buds of different ages. Fig. 55 shows 

 a bud still more enlarged, with the proboscis (manubrium) just 

 formed, and knob-like, rudimentary tentacles. In an older stage 

 (Fig. 56) the proboscis is enlarged and the tentacles lengthened, 

 while the depression at the upper end indicates the future opening. 

 In Fig. 57 the appendages of the proboscis are plainly indicated, 

 the tentacles are turned outwards. Shortly after this the jelly-fish 

 breaks loose from its attachment and swims around as at Fig. 58. 



How do the zooids first arise ? This leads us to speak of the sim- 

 plest mode of reproduction in the Hydroids, which is by budding. 

 The object of sexual reproduction, i. e., by eggs and spermatozoa, 

 throughout the animal and plant world, is by bringing the germ or 

 portion of protoplasm of one individual, which is an epitome 

 potentially of its physical and psychical nature, to mingle with 

 that of anotler of the same species, so that the offspring may 

 combine the qualities of both parents* and not deteriorate. T ue 



