CHAPTER II. 



descriptions of the genera and species of invertebrates 



found in the marcellus, hamilton. genesee, and 



naples beds of eighteen milk creek and the 



lake-shore region of erie county, new yokk. 



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 mewglcea, 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 go nopolyps, which produce 

 jelly-fish or medusae. These latter 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 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, seaw T eeds, 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 Palaeontology ( Eastman's translation ), 

 where an extensive bibliography is given. 



Genus DICTYONEMA. Hall. 



[Ety. : Dictyon, net ; nema, thread.] 

 (Pal. N. Y., Vol. II., p. 174.) 

 Colony forming 1 a network of anastomosing branches, the 

 whole commonly flattened on the rock, but originally form- 

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



