MORPHOLOGY. 405 



extent.* In the echinoderms there is a repetition of 

 ambulacra and ambulacral plates, but this is supposed 

 to be different in its origin from that in the segmented 

 animals. 



Study of different animals further reveals the fact that 

 in some there is no cavitv inside the body aside from that 



/ / 



of the digestive tract (sponges, ccelenterates), while in all 

 others there are more or less well-marked cavities between 

 the alimentary canal and the body-wall. Further and 

 more detailed studies lead to the conclusion that there 

 may be at least three different categories of cavities in 

 the body, those of the excretory organs, those of the circu- 

 latory organs, and a third, the coelom, sometimes small 

 and containing only the reproductive glands, sometimes 

 large, as in the vertebrates, and including not only the 

 reproductive organs, but other structures as well. So the 

 term ' body-cavity / often used, is inexact. The body- 

 cavity of a lobster, for example, is merely an expansion of 

 the circulatory system and has no relation to the body- 

 cavity of the fish or frog, which is a true ccelom. Thus 

 the large cavity of the echinoderm and that of the am- 

 bulacral system, the pericardium of the mollusc, the 

 paired ' body -cavities' of the annelids, the cavities of the 

 reproductive organs of the arthropods, and the pleuro- 

 peritoneal and pericardial cavities of vertebrates are all 

 coeloms. 



All animals reproduce sexually, as mentioned on p. 141, 

 but besides this sexual reproduction, many animals pos- 

 sess the power of reproducing asexually. In these cases 

 the animal may divide into two (fission), or a small por- 



* The gills, kidneys, and heart of the Chitons (p. 197) and the 

 Nautilus (p. 211) are supposed to present indications of segmenta- 

 tion. 



