The spermatogenesis of Hydra. 381 
preceding actual division; appreciation of the purpose of multipli- 
cation of the interstitial or germ cells, namely to increase the 
number of sperm; and that the head of the sperm is the cell 
nucleus. Such was the condition of our knowledge of spermato- 
genesis among the coelenterates when the present investigation was 
undertaken. 
Species of Hydra. 
The spermatogenesis of three species of Hydra has been studied. 
In undertaking to discover which species these were, a tangle of 
terminology has been encountered. NussBAum, 1887 (70) after care- 
fully reviewing the literature on the various species of Hydra con- 
cludes that all the forms described can be included under four 
species: viridis, grisea, fusca, and attenuata. He says however that 
he cannot vouch for the latter species as he has not seen it. It is 
one described by Röser (73) and Parzas (71). Braver, 1891 (5) also 
recognizes four species, but not the same ones as Nusspaum gives, 
for he drops attenuata and points out that under H. fusca have 
been included two species, fusca proper, and a species which he 
indicates as “spec?” He demonstrates its right to a spearate specific 
name, for it alone is unisexual and moreover is characterized by 
differences of structure, gross anatomy and peculiarities of the egg. 
Hareirr, 1901 (30) says in the synopsis of the hydromedusae that 
“of the genus Hydra there are two well distinguished species, H. fusca 
and H. viridis”. 
We have found four species and have studied the spermato- 
genesis of all but grisea. Combining the specific characters esta- 
blished by Jicken1, Nusspaum, BRAUER and others, these species 
would be as follows: 
I. Hydra viridis L. Synonomy: Polype verd, TREMBLEY: grüner 
Polyp, Rösen; Hydra viridissima, Pauuas. Body green. Tentacles 
5—12, seldom as high as 18; shorter than the body. When body 
is stretched it is 1—1,5 cm long. Nettle cells small. Hermaphro- 
dite; sexually mature April—Oct. Eggs fall off from parent. Egg 
spherical and its sheath almost smooth. 
A smaller variety, H. viridis var. bakeri is found in salt marshes. 
II. H. grisea L. Synonomy: Polype de la seconde espèce, 
TREMBLEY; oraniengelber Polyp mit langen, hörnerförmigen Armen, 
RoseL; H. vulgaris, H. aurantica, H. grisea, Leypic and KLEINEN- 
BERG; H. trembleyi, HaacKE. Gray, pale yellow or reddish. (The 
color, except ini. viridis, is so dependent on food that no reliance 
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