HYDROIDS. 



87 



M 



.V. 



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 



ncmatophores, are filled with processes of the body substance, 



and in structure and development are believed by liamann 



to give evidence of being degenerate zooids. Certain of the 



branches or piiina3 are at times replaced by cylindrical struc- 



t arcs which are covered with rows of ncmatophores, and are 



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



<il)ed ; they are termed corbula^ and in some genera are meta- 



n!or]ihosed branches, while in others they are modified pinna'. 



\ jiinna is smaller than a branch, and differs from it in the 



character of the zooids formed upon it. The egg develops 



into a planuin, wliieh becoming attaclied forms a new h3-droid 



colony. 



These three great families, rejiresented here by the genera 



tSertularkt, Obella, Goiiotliijrea, and Agluopheiua, 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 Ilineks, CalyiJtobhistea, bv 



Allman, and Intaiiiiolata by Hamann. 



As among the Gymnoblastea, we find here medusw which 



agree in structure with those which are undoubtedly calyjito- 



blastic, but of whose early development we know nothing. 



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

 fishes is Zij(jodactyla ijronlundica., which sometimes acquires 



:\ diameter of even eleven inches. In color it is a light violet, 



with numerous brownish reproductive organs. The numei'ous 



tentacles which fringe the margin of the umbi'ella hang down 



a yard or more when fully extended. Concerning the habits 



of tliese 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 contract ion of 



the disk, but in the Zi/(jodact[/la these undulations have a. 



certain •graceful 

 indolence, very 

 unlike the more 

 r ;i ] 1 i d m o v e- 

 mi'uls (jf many 



of the medu.sa?. It often remains quite 

 motionless for a long time and then, if 

 y<iu try to excite it by disturbing the 

 w.'itcr 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 

 relai>ses 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. 



A^ 



P- 



X 



/,/„ 



.0. — Corbulaof A<il<ui~ 

 strtithionUkH, enlarged. 



\?l<i. 81. — Zi/f/odaffifla r/rijidanctica. 



