22 NATURAL HISTORY OP 



Flustra papyracea, Cellariajistulosa, Alcyonidium para- 

 siticum, A. mamillatum. 



Deep-water zone. Tuhularia simplex, T. attenuata, 

 T. indivisa, Lafoea fruticosa, Halecium muricatum, H. 

 lahrosum, Sertularella Gayi, S. tricuspidatum, 8ertu- 

 laria ahietina, 8. fusca, Campanularia gracillima, 

 Diphasia pinaster, Thuiaria thuja, T. articulata, I/yto- 

 ca.rpus myriophyllum, Plnmularia Gatharina, Tealia 

 digitata, Diastopora ohelia, Stomatopora major, Cellepora 

 pumicosa, C .ramulosa, Palmicellaria Skenei, P.cribraria, 

 Smittia trisp)inosa, S. reticulata, Schizoporella linearis, 

 Menipea ternata, Mucronella Peachii, Bugula Murray- 

 ana, -Retepora Beaniana. 



HYDKOZOA. 



I. Hydroida. 



Description of the Structure of the Hydroida. 



In order that a clear idea may be conveyed of the 

 structure of the Hydroida, it will be well to describe 

 fully that of the most easily accessible member of the 

 order, the common fresh-water polyp or hydra; and 

 from that description to trace the formation of the 

 fixed formSj which may be looked upon as compound 

 hydrse. In Plate I. fig. 1, will be found a diagram of 

 the structure of the hydra, from which it will be seen 

 that the animal consists of a gelatinous body or sac, 

 one end of which, called the proximal end, is expanded 

 into a disc or foot, whilst the other end or oral ex- 

 tremity, called the distal end, is occupied by the mouth 

 {m) and tentacles (t). The form of the body of the 



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