HYDROIDS. 



87 



pointed teeth, and three minute tubular processes are disposed about its mouth, one 

 on each side and one on the outer or anterior surface. These processes are termed 

 nematophores, are filled with processes of the body substance, 

 and in structure and development are believed by Hamann 

 to give evidence of being degenerate zooids. Certain of the 

 branches or pinnae are at times replaced by cylindrical struc- 

 tures which are covered with rows of nematophores, and are 

 the cups or baskets in which the generative zooids are devel- 

 oped ; they are termed corbulse, and in some genera are meta- 

 morphosed branches, while in others they are modified piunaj. 

 A pinna is smaller than a branch, and differs from it in the 

 character of the zooids formed upon it. The egg develops 

 into a planula, which becoming attached forms a new hydroid 

 colony. 



These three great families, represented here by the genera 

 Sertidaria, Obelia, Gotiothyrea, and Aglaophenia, are all mem- 

 bers of a sub-order of hydroids distinguished by having the 

 hydranths surrounded by chitinous cups, and the possession 

 of longitudinal ridges in the body cavity. This group has 

 been variously termed Thecata by Hincks, Calyptoblastea by 

 Allman, and Intseniolata by Hamann. 



As among the Gymnoblastea, we find here medusae which 

 agree in structure with those which are undoubtedly calypto- 

 blastic, but of whose early development we know nothing. 

 We can mention but one example. One of our larger jelly- 

 fishes is Zygodactyla gronlandica, which sometimes acquires 

 a diameter of even eleven inches. In color it is a light violet, "V^ ""'-'A/ 

 with numerous brownish reproductive organs. The numerous 

 tentacles which fringe the margin of the umbrella hang down 

 a yard or more when fully extended. Concerning the habits 

 of these animals Mrs. Agassiz has written : — " The motion of 

 these jelly-fishes is very slow and sluggish. Like all of their 

 kind, they move by the alternate dilation and contraction of 

 the disk, but in the Zygodactyla these undulations have a 



certain graceful 



indolence, very 



unlike the more 



rapid m o v e- 



ments of many 



of the medusas. 



Fig. 80. — Corbula of Af/lao- 

 plicnia struthionides, enlarged. 



It often remains quite 

 motionless for a long time and then, if 

 you try to excite it by disturbing the 

 water in the tank, or by touching it, it 

 heaves a slow, lazy sigh, with the whole 

 body rising slowly as it does so, and then 

 relapses into its former inactivity. In- 

 deed, one cannot help being reminded, when watching the variety in the motions of 

 the different kinds of jelly-fishes, of the difference in temperament in human beings. 



Fjg. 81. — Zygodactyla gr'dnloAuliea. 



