NIAGARA FALLS AND VICINITY 1 33 



ANIMALS 

 Class HYDROZOA Owen 



This class includes the simplest polyps, of which the fresh-water 

 Hydra is an example. The body consists of a hollow tube, the walls 

 of which are composed of two cellular layers, ectoderm and endoderm, 

 with a non-cellular layer, the mesogloea, between them. These lay- 

 ers meet at the mouth, which is the only opening into the gastric 

 space inclosed by the body wall. Tentacles, furnished with nettle 

 cells, surround the mouth. 



A few hydroids are simple forms, but the majority are united into 

 colonies, which frequently assume a branching or tree-like char- 

 acter, a polyp occupying the end of each branch. Reproduction is 

 usually carried on by specially modified polyps, the gonopolyps, 

 which produce jellyfish or medusae. These may remain attached 

 to the colony or become free-swimming. 



Some hydroids are entirely unprotected, no hard structures being 

 developed, and these consequently leave no remains. The majority 

 of species, however, secrete a horny or chitinous covering, the peri- 

 derm, which invests the whole stock, and in one group is expanded 

 at the ends of the branches into cups or hydrothecac, into which the 

 polyps can withdraw. This chitinous periderm may be preserved 

 in the form of a carbonaceous film (e. g. Dictyonema and 

 Graptolites). 



Some hydroid colonies, i. e. the hydrocorallines, secrete at the 

 base a dense calcareous covering, which has much the aspect of 

 coral, and is frequently mistaken for that (e. g. Millepora, 

 Stromatopora). M ost hydroid colonies are permanently at- 

 tached to rocks, seaweeds, or other objects of support. 



Genus dictyonema Hall 



[Ety. : oiv-oov, net; vrjiia, thread] 



(1852. Pal. N. Y. 2:174) 



Colony forming a network of anastomosing branches, the whole 

 commonly flattened on the rock surface, but originally forming a 

 funnel or fan-shaped expansion. The branches proceed from a 

 common acute base, divide frequently, and are at intervals united 

 again by transverse dissepiments. The outer surfaces of the 

 branches are striated; the inner bear hydrothecae, though these are 

 seldom seen in the flattened specimens. 



Dictyonema retiforme Hall. (Fig. 27) (1852. Pal. N. Y. 2:174, 

 pi. 40F) 



