22 A Guide to the Zoological Collections in the 



differently occupied and variously modified " persons " are 

 indicated by letters on the life-sized models in Case 3. 



Such colonies, made up of units of many different forms, 

 are known as Polymorphic Colonial Hydrozoa, 



Almost all the Hydrozoa are marine, Hydra being the 

 best known of the very few freshwater forms. 



The fixed Hydrozoa usually live attached to rocks or 

 encrusting sea-weeds, dead shells, etc., between low water 

 mark and lOO fathoms; a few are known from the deep 

 sea ; while some species attach themselves to freely loco- 

 motive animals, such as crabs, fishes, etc., by which arrange- 

 ment they greatly profit. The free-swimming Medusas 

 are found at the surface of all the seas and oceans. 



Not to include the usual glass models, the Hydrozoa 

 are represented in the exhibited collection by spirit-speci- 

 mens of the following genera : Tubtilaria, Eudendrium, 

 Corydendrium, Cordylophora, Olindias, Sertularia, 

 Plumularia, A nt ennui aria , only the last three being from 

 the Indian Seas, 



2, 3, 4. CNIDARIA ANTKOZOA, SCYPHOMEDUS/E, AND 

 CTENOPHORA. 



[Cla0£0 4—16]. 



In the second division of the Cnidaria or Stinsfine 

 Zoophytes the mouth is formed, as in all higher animals, 

 by an invagination of the ectoderm. The mouth, therefore, 

 does not open directly into the gastric cavity as it does 

 in the Hydrozoa, but into an ectodermal invagination, or 

 gullet, which leads to the gastric cavity or stomach. 



There are three distinct and equivalent types or Classes 

 of these more highly developed Cnidaria, (i) the Anthozoa- 

 type, of which the Sea- anemones and Coral polyps are 

 good examples, (2) the Scyphomedusa-ty^e, of which the 

 large Acraspedote Medusa are the examples, and (3) the 

 Ctenophor type. 



In the typical Anthozoan the form of the body is still a 

 three-layered tube usually closed and fixed at one end, free 



