CiiAr. ir. 



THE DIFFERENT RADIATA. 



49 



Fi'j. 24. 



terminating with a larger Polyp, -wliicli is perfectly symmetrical; while the individuaLs 

 which stand upon the sides of the branches are not only smaller but nt the same 

 time one-sided, the broader and more prominent side being turned outward, and 

 the tentacles on that side being also larger than those turned toward the com- 

 mon axis. 



Among the Hydroids, as among the Polyps, we find those in Avhich the com- 

 munities are formed l^y identical individuals diflering, perhaps slightly, in size. 

 This is the case in the flimilies to which the genera Tubularia (Vol. IV., PI. XXIV.) 

 and Coryne (PI. XVII., XVIII., and XIX.) belong. But there are others, in which 

 we find, either constantly or at least at certain seasons, two kinds of individuals, 

 diflering not only in size, but also in form, and still ftirther in the presence or 

 ahsence of tentacles, one kind being always sterile, while the other produces MedusiB 

 buds that may be freed. This is the case with the Campanularians {Figs. 10, 15, 16, 

 and 17) and the Sertularians {Figs. 18, 22, and 23). In the Plumularians, the diifer- 

 ences are still more marked ; for besides the fertile indi- 

 viduals there are several kinds of sterile individuals, 

 groujjed together in various clusters, the smaller ones 

 being attached around the large ones. Finally, there 

 is a genus — Hydractinia [Figs. 21 and 2o) — which, among 

 the Hydroids, exhibits the greatest range of difference 

 thus far observed between the individuals of the same 

 species. For in this genus we have, in the first place, 

 tw^o kinds of communities : one {Fig. 25) in which the 

 fertile individuals produce only male Medusa?, and another 

 {Fig. 24) ill which the fertile individuals produce only 

 female Medusa?. Again, the fertile 

 individuals in both kinds of com- 

 munities have tentacles {Figs. 24 o 

 and 25 h, o) entirely different from 



those of the sterile individuals. The sterile individuals {Figs. 

 24 a and 25 a) differ also greatly among themselves, some being 

 slender and almost thread-like; others slender, but with a dis- 

 tinct proboscis and a whorl of tentacles; others short, widening 

 greatly upward, and assuming almost the form of a trumpet- 

 mouth. All these individuals differ not only in their form 

 and complication, but also in their color, so that we have in 

 this genus about as great a diversity of individuals in one 

 community, as is observed in the most complicated 8iphono- 

 phora?. The only difference between the two groups consists in this: that while all 



Fifj. 25 



HyDKACTINIA rOLYCLIXA, Ag. 



sterile individual. — b Fertile individual pro- 

 duciu}; female Medusie. — (I e Female Medus;e 

 containing advanced eggs. — fi^hi Cluster 

 of female Medusje witli less advanced eggs. — 

 I'eduncle of the moutli with short globular 

 tentacles. — c Individuals with globular ten- 

 tacles, upon which no Medusa? have as yet ap- 

 peared, or from which they have already 

 drojtped. 



Hydractinia polyclixa, Ag. 



aa Sterile individuals. — b Fertile 

 individual, producing male 51e- 

 dusa^. — d Clusters of male Me- 

 dusae. — Proboscis, witli the 

 mouth at the apex. — t Elongated 

 tentacles of the sterile individu- 

 als ; in the fertile one 6, they are 

 simple knobs upon the proboscis o. 



