SURVEY OF INVERTEBRATES 



73 



feeding polyps, or hydranths, and the medusoid polyps which 

 later, with some structural changes, are set free. Again in Obelia, 

 another common Hydroid, there are also highly modified individ- 

 uals, or gonangia, that produce by budding the medusa generation. 

 In still other Hydrozoa this polymorphism is carried even further. 

 Thus Physalia, the Portuguese Man-of-War, is a floating Hydroid 

 colony consisting of an air-filled bag, or float, with a sail-like crest, 

 from which are suspended a large number of polyps. These indi- 

 viduals are very diverse: some are nutritive, others are tactile, 

 and still others bear batteries of nematocysts. Furthermore, there 

 are male reproductive polyps and others that give rise to egg- 



Ectoderm 



Ectoderm 



Enteric 

 cavity 



Tentacle 



Enteric 

 cavity 



Radial canal 



\ Circular 

 canal 



Fig. 38. — Diagram showing the fundamentally similar structure of Hydra 

 or a hydranth of Obelia (A) and of a medusa (B). (From Parker.) 



producing medusae. Obviously an animal such as Physalia sug- 

 gests that in many of the lower forms the individual is not so 

 sharply defined, and is more difficult to define, than in the more 

 familiar animals. Indeed, individuality in the Animal Kingdom 

 is a problem in itself; one the reader might well keep in mind as 

 we ascend the scale of animal life. 



Scyphozoa. The second great class of the Coelenterates com- 

 prises chiefly large medusae, such as the common saucer-shaped 

 Aurelia of the Atlantic coastal waters or its giant relative, Cyanea, 

 which may attain a diameter of eight feet; each and all well- 

 named Jellyfish since their soft tissues are more than 99 per cent 

 water. 



The structure of Aurelia is basically the same as that of the 

 medusa generation of the Hydroids; the most obvious difference 

 being an excessive development of the mesogloea between ecto- 



