170 Frof. Allman on the Hydroid Zoophytes. 



extend even to the extremities of the two tentacles, and by whose 

 aid the zooid swims actively in the surrounding water. 



The endoderm of the tentacles presents very distinctly the 

 septate condition so frequent in the tentacles of the Hydrozoa; 

 and by carefully managed compression this peculiar structure 

 can be plainly seen to be continuous witli a similar one in the 

 endoderm of the spadix, where, however, it is looser and less 

 regular, and does not encroach upon the central cavity. The 

 cavity of the spadix is lined with a thick glandular layer whose 

 cells are filled with reddish-brown matter. 



In the male zooids the spadix is surrounded by a continuous 

 mass of spermatozoa ; in the female, the place of the sperma- 

 tozoa is occupied by ova, which are invariably two in number, 

 and are placed one on each side of the spadix, and in such a 

 position that only one is visible when the zooid is viewed in the 

 plane of the two tentacles. 



The spermatozoa are caudate corpuscles, the corpuscle having 

 the form of a cone, with a broader base than is usual among the 

 Hydrozoa. 



The ova pi-esent a well-defined germinal vesicle, in which a 

 germinal spot is visible or may be easily rendered so by slight 

 compression. They are invested by a proper membrane, which 

 is remarkably thick and richly studded with thread-ceils — a 

 character of which I am not aware of any other instance among 

 the Hydrozoa. 



The sac which contains the generative elements is simply the 

 endotheca of the original gonophore ; its walls contain abundance 

 of thread-cells. The narrow posterior portion from which the 

 tentacles spring represents the base of the manubrium, with the 

 ectoderm and endoderm not separated from one another by the 

 intervention of ova or spermatozoa. 



The sexual zooid of Dicoryne conferta always swims with its 

 body in a vertical position, carrying the posterior or tentacular 

 extremity uppermost, and maintaining all the time a constant 

 rotation on its longer or vertical axis. 



If we now compare the sexual zooid of Dicoryne conferta with 

 the medusoid of other species, we shall find that it corresponds 

 exactly to an ordinary medusoid reduced to the manubrium, but 

 having the place of the umbrella occupied by two tentacles. 

 The tentacles themselves are, I believe, the representatives of 

 the radiating canals of the medusoid. It will be recollected that 

 they are turned forward while the zooid is still enveloped by 

 the ectotheca; and it may also be borne in mind that in certain 

 medusoids, as those, for example, of Laomedea dichotoma, the 

 umbrella is frequently inverted and thrown back, so as to 

 assume the position of the tentacula in the free zooid of Di- 



