No. 412.] MORTH-AMERICAN INVERTEBRATES. 303 
impossible to formulate any scheme of classification which 
shall bring into a single view the complete ontogenetic rela- 
tions of the various hydroid and medusoid phases. In the 
present synopsis I have followed in the main that of Allman 
and Hincks for the hydroids and that of Haeckel for the medusz, 
though in each there is not a little variation both as to the order 
of presentation as well as the nomenclature used.! 
SYNOPSIS OF ORDERS OF HYDROMEDUS-€. 
I. HypDRARIA. 
Polyps solitary, never forming colonies; no medusoids; sex-cells pro- 
duced in ectoderm of polyps. 
Of this order only a single well-defined genus is recognized, Hydra. 
The genera Protohydra and Microhydra are probably allied genera, but 
their affinities are too uncertain as yet to warrant definite classification. 
The former is of marine habit, the latter of fresh-water habit. Both are 
devoid of tentacles, and sexual reproduction, at least in the former, seems 
unknown. 
Of the genus Hydra there are two well-distinguished species: HW. fusca 
and 7. viridis. Both abound in fresh waters of small lakes, ponds, and 
sluggish streams, associated with various aquatic plants, notably Lemna, 
various algze, pond lilies, etc. 
Il. HypROCORALLINA. 
Colonial. Hydrosome comprising polyps of two forms, gastrozoids and 
dactylozoids, supported from a network of ccenosarcal hydrorhize, from the 
ectoderm of which is secreted a calcareous mass which is deposited over 
the spaces or meshes of the network. The colonies form incrusting, often 
arborescent, masses over shells, stems of Alcyonaria, or other support, 
often forming massive and fantastic shapes, as in the so-called “stag’s 
horn coral.” Only one genus is likely to come within the range of 
the present synopsis, namely, Millepora; and of this a single species, 
alcicornis. 
III. TusuLarr# (Gymnoblastea). 
The Tubularie are for the most part colonial hydroids, producing free 
medusz, or medusoid gonophores, by budding. Hydroids devoid of 
1 Just as these notes were being put into final form for the press, I have been 
permitted to consult the manuscript of a forthcoming Handbook of the Hydroida of 
the Woods Hole Region, by Professor C. C. Nutting, for the privilege of which I am 
under grateful obligations. 
