A Guide to the Zoological Collections in the 



I. CNiDARiA HYDRO MEDUSiC. 



[OTas^s 3—4]. 



In this great Class the type is an animal having the form 

 of a simple tube closed and often attached at one end — the 

 aboral pole ; and provided at the other end (oral pole) 

 with a mouth surrounded by tentacles. The tube of which 

 the animal consists is mainly formed of two layers, — an 

 outer layer or ectoderm, and an inner layer or endoderm — 

 and between these two there occurs the elements of a 

 third supporting layer, or mesoderm. The hollow of the 

 tube is the gastric cavity, or stomach. 



This simple type is seen in Hydra, an animal that can 

 only be properly examined under the microscope, and 

 which is therefore exhibited in the form of an enlarged 

 drawing. 



This simple type becomes complicated by the fact that 

 the parent polyp usually buds and branches again and 

 again — the resulting offspring remaining united together 

 and to the parent to form a tree-like colony. 



This is well seen in the exhibited specimens of Tiibu- 

 laria, Eudendrium, Corydendrium, Plumularia, and Ser- 

 tularia. 



In these colonial forms the units of the colony are 

 known as "zooids," and the fleshy substance which binds 

 them together is known as the "coenosarc." In the ecto- 

 dermal coenosarc a protective and supporting investment 

 is often secreted. In Tubularia and the allied forms this 

 is a chitinous pellicle. In Perigonimus the chitinous pelli- 

 cle is strengthened by grains of sand. In Plumularia 

 and Sertularia the chitinous investment is very thick 

 and stout, and is moreover continued over the zooids up 

 to the level of their tentacles to form protective cups 

 into which the zooids can completely retract themselves 

 when alarmed. In Millepora, instead of a membranous 

 or chitinous pellicle, a hard stony deposit, in structure ex- 

 actly similar to a stony coral, is formed by the ectoderm. 



