22 THE OCEANIC HYDROZOA. 



polypite, with or without tentacles, suspended from the roof of a bell-shaped body, having the 

 structure of a nectocalyx, and like it, provided with a marginal valve, and with radiating 

 and circular nectocalycine canals. The reproductive organs are usually simple spermaria 

 or ovaria, developed in the walls of the canals or in those of the polypite. 



SECT. III. GENERATION AND DEVELOPMENT OF THE HYDROZOA. 



The substance of the spermarium in all \X\e Hi/drozoa in which I have traced its develop- 

 ment (and I have studied to this end members of each of the orders) becomes differentiated 

 into minute, clear, spheiical vesicles, of about ^^jth to ^jjlioth part of an inch in diameter, in which 

 I have been able to observe no further structure. The vesicle gradually becomes pointed at 

 one end, while at the other it developes a slender cilium, and, eventually, the vesicle assumes 

 the characters of the head of the spermatozoon, while the cilium becomes its tail. The 

 head of the spermatozoon is always broader at that end to which the cilium is attached. 



The ovarium, in like manner, breaks up into a number of spheroidal, or originally, 

 polygonal masses, each of which contains a clear space, with or without a distinct wall — the 

 germinal vesicle, and in its centre a smaller, thick- walled vesicle — the germinal spot. I have 

 never observed any vitelline membrane around these ova. Allman describes none in 

 Cordyloyhora, and Gegenbaur (p. 49) distinctly denies the existence of any in the ripe ova of 

 those Calycophoriche and Physophoridce which he examined. 



Gegenbaur (p. 49) has observed the direct contact of the spermatozoa with the ova 

 in the Cali/cophoridce and Fhysophoridm. The spermatozoa never entered the ovisacs, 

 but as soon as the ova were detached they were surrounded by the spermatozoa, which fixed 

 themselves by their heads to the yelk, their vibrating tails radiating in all directions. 

 Gegenbaur, however, does not appear to have witnessed the direct penetration of any 

 of them into the substance of the yelk. Complete yelk-division takes place in the ordinary 

 way in all Hydrozoa, and Gegenbaur has made the important observation that, in certain 

 Corynida} CalycophoridtB and P/iysophoridcB, the germinal vesicle does not disappear, but that 

 its division immediately precedes that of the yelk — so that its progeny must eventually 

 become the " embryo cells " of the division masses. 



Towards the end of the process of yelk-division, cilia appear upon the surface of the 

 embryo. 



Thus far all the Hydrozoa appear to follow a like course, but from what is at present 

 known it would seem that the different orders diverge somewhat in their further progress. 



In Tuhularia and Cordylophora, in Campanularia {?) in Cyancea (and hence, probably, in 

 all Corynida, Sertulariada, and Lxiceriiariada), a cavity rapidly makes its appearance in the 

 centre of the germ-mass, which thus becomes a blastodermic vesicle, and then the walls 

 of this vesicle are differentiated into ectoderm and endoderm. 



From Gegenbaur's observations it would appear that in the Calycophorida and Physo- 



See the account of the development of the ova of the Coryiiidan medusiform zooid, Oceania 

 armata, in Gegenbaur's instructive Essay, ' Zur Lehre vom Generatious-wechsel,' 185-1. 



