240 C'CELENTEEATA. 



The development of the ovum, which is, as a rule, naked (i.e., with- 

 out a vitelline membrane), has hitherto only been completely followed 

 out in a few cases. In every case the segmentation seems to be com- 

 plete, and leads to the formation of a segmentation cavity and a 

 single-layered blastoderm [a single-layered blastosphere]. The 

 latter gives rise to a second endodermal layer of cells, which lines 

 the segmentation cavity. The segmentation cavity thus becomes 

 converted into the gastric cavity of the future polyp. The spherical 

 or oval larva now either attaches itself and gives rise by budding 

 to a small hydroid stock, or swims freely and develops directly into 

 a small Medusa (TrachymeduAce). 



The Medusa, after becoming free, usually undergoes a more or less 

 fundamental change of form, which concerns not only the alteration 

 caused by the enlargement of the umbrella and manubrium, but also 

 the increase, according to definite laws, of the marginal tentacles, 

 sense organs (Tima), and the radial canals (Aequorea). We must 

 remark, however, that the sexually complete Medusae exhibit very 

 considerable variations in size, number of sense organs and tentacles 

 (Phycdidiwn variabile, Clythia volubilis}. 



The difficulty of systematic arrangement is augmented by the fact 

 that closely allied Polyp stocks can produce different sexual forms. 

 Thus, for example, Monocaidu* gives rise to sessile generative buds 

 and Corymorpha to free Medusce (Steenstrupici). Medusae of identical 

 structure also, which one would place in the same genus, may form 

 the sexual generations of hydroid stocks belonging to different 

 families (isogonism). There are also cases in which we find Medusa 3 , 

 of closely allied genera, some developed from hydroid stocks by an 

 alternation of generations, and others developed directly. Hence 

 it appears just as little satisfactory to found a classification entirely 

 upon the sexual generations as to pay attention to the asexual 

 generation alone. 



(1) Sub-order: EleutheroUastece. Simple hydroid Polyps without 

 medusoid buds ; both generative products are developed in the body- 

 wall of the Polyp. 



Fam. Hydroidoe. Hydra, the fresh- water polyp. II. viridis L., H.fusca L., 

 remarkable for great powers of reproduction. 



(2) Sub-order : Hydrocorallice. Coral-like hydroid stocks with cal- 

 careous coenenchyma and tubular hydrothecse opening to the exterior 

 by pores. Some of these contain the larger nutritive animals, while 

 others contain animals without a mouth and beset with tentacles. 



