130 UNIVERSALITY OF DEGENERATIVE EVOLUTION 



tion of all the organs not connected with reproduc- 

 tion. Degeneration has made furthest advances in 

 Bonellia, which affords a striking example of retro- 

 gression side by side with development. 



Investigations of animal series such as these 

 might well be continued throughout the various 

 classes and groups, showing the existence of rudi- 

 mentary organs in all. We will restrict ourselves 

 here, however, to mentioning the larger subdivisions 

 only, taking one example from each group. 



3. The Echinoderms. This order comprises 

 star-fish or asterids, sea-urchins, Crinoids and 

 Holothuria or trepangs. 



Of these we will take the star-fish, and proceed 

 to examine its digestive tube. Under normal 

 circumstances, the intestine terminates in a dorsal 

 anus, centrally, or slightly excentrically placed, pre- 

 ceded by a very short but well-developed rectum. 

 Asteracanthion and- Solaster furnish good examples 

 of this. In some kinds of Asterids in Astropecten 

 aurantiacus, for instance the anus no longer exists, 

 and the rectum, having become useless, is greatly 

 reduced, though still exhibiting signs of its original 

 condition. 



4. Mollusca. Of the group of Mollusca we 

 will take the Gastropods and the Cephalopods as 

 examples. 



There are two kinds of Gastropods, straight 

 and twisted, the former representing the primitive 

 type. The straight types such as Chiton, Patella, 



