('HAPTKK II. 



descriptions of the genera and species of iwektebkates 



found ix the marcellt's, hamilton, (4enesee, and 



naples heds of eighteen mile creek and the 



lake-shore relilon of erie cot'ntv, new york. 



Class Hydrozoa. Owen. 



This class iucludcs 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 endodenn. 

 with a non-cellular layer, the inesoghva, between them. These layers 

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

 enclosed by the body wall. Tentacles, furnished with nettle-cells, sur- 

 round the mouth. 



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

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

 polyp occupying the end of each branch. Reproduction is usually 

 carried on by specially modified polyps — the gonopoh/ps, which produce 

 jelly-fish or medusae. These latter may remain attached to the colony 

 or become free-swimming. 



Some h3'droids are entirely unprotected, no hard structures being 

 developed, and these, consequently, leave no remains. The majority of 

 • species, however, secrete a lioi'ny or chitinous covering — the periderm, 

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

 of the branches, into cups or hydrothecce into which the polyps can with- 

 draw. This chitinous periderm may be preserved in the form of a carbo- 

 naceous film. 



Some hydroid colonies secrete a calcareous covering which has much 

 the aspect of coral, and is frequently classed as such (e. g., Millepora). 

 Most hydroid colonies are permanently attached to rocks, seaweeds, or 

 other objects of support. 



Note. — For a detailed account of the structure of living Hydrozoa, see 

 any advanced text-book of zoology or anatomy. The fossil genera are 

 discussed in Zittel's Text-book of Palaeontologj" (Eastman's translation), 

 where an extensive bibliography is given. 



Genus DICTYONE^IA. Hall. 



[Ety. : Dirt you, net ; nema, thread.] 



(Pal. X. Y., Vol. H., p. 174.) 



Colony foniiiii^- a network of an;istomosin<;' br.inches, the 



whole coninioiily flattened on the rock, but originally form- 



ingafnnnel- or fan-sha])e(l expansion. The ln-anohes ])i'oceed 



