THE GROUPS OF ANIMALS 



263 



drawn apart so that the body is a long cyUnder hke a polyp (also see 

 Fig. 143). The medusa is regularly free-swimming, though because it is 

 produced by budding from the polyp form it remains in some species 

 attached to its parent. The polyp is usually sessile, though sometimes, 

 as in Hydra, it may become detached from one object and loop along 

 to a new situation where it again glues itself fast. 



A BCD 



Fig. 219. — Various coelenterates: A, Gonionemus; B, Aurelia; C, sea anemone; D, coral. 

 {A-C from Carolina Biological Supply Co.; D from Wolcott, "Animal Biology.'') 



Colony formation is common. Most of the hydroids are branching 

 colonies. The corals have massive stony skeletons which in the aggregate 

 may form reefs and atolls or other islands. The sea pens are colonies 

 resembling a quill pen, with the pointed end thrust into the sand. Many 

 of the colonial types are gorgeously colored and are responsible for some of 

 the brilliance of tropical seas. The siphonophores (Figs. 145, 146, pages 

 174, 175) are free-swimming colonies. 



Coelenterates exhibit a great deal of polymorphism. The polyp 

 and medusa have already been mentioned as generalized types. Each 

 may be considerably modified in different species and 

 modified in several different ways in the same species. 

 In the hydroids the medusa shows more variation than 

 the polyp. It is free-living in some species, perma- 

 nently attached to the hydroid colony in others. When 

 attached, it may suffer considerable reduction; that is, 

 it does not develop the full medusoid structure, which 

 -would be useless to an inactive individual. Sometimes 

 the reduction of the medusa is so great that practically 

 only the gonads are left (Fig. 220). Then the medusa 

 looks like a reproductive organ belonging to a colony 

 of polyps. 



Much more marked polymorphism is found in the siphonophores 

 (page 174). In them there are usually several kinds of structures which 

 betray, sometimes in vague but often in unmistakable ways, their 

 medusoid architecture and several other kinds which, in development or 

 adult anatomy, are more or less like the polyp. 



These polymorphic species often show that type of alternation of 

 generations which is kno^vn as metagenesis (page 174). One or more 



Fig. 220.— Hy- 

 droid, with re- 

 duced medusae. 

 {Courtesy of Caro- 

 lina Biological 

 Supply Co.) 



