164 THE PHILOSOPHY OF BIOLOGY 



tinuously from the posterior part of the scolex, and 

 they may remain in connection with each other, and 

 with the central nervous system and some other 

 organs which are concentrated in the scolex. Never- 

 theless, each proglottis contains a complete set of 

 reproductive organs ; it has locomotory organs so that 

 it can move about, and can fix itself to any surface 

 into which it comes in contact. It can lead, for a 

 considerable time, at least, an independent existence 

 apart from that of the scolex and the other proglottides 

 with which it was originally in continuity. In the 

 majority of Polyzoa, the common Sea-Mat, for instance, 

 the organism consists of a very large number of polypes 

 or zooids, each of which secretes an investment of 

 some kind round itself, but all of which may be con- 

 nected together by a common flesh. In many Zoo- 

 phytes there is essentially the same structure. In 

 Corals there are very numerous zooids, each of which 

 lives in a calcareous calyx secreted by itself. Polyzoa, 

 Zoophytes, and Corals are individuals of the third 

 order, and we might regard the tapeworm strobila 

 the scolex with its chain of proglottides as belonging 

 also to the same category. Nevertheless, a part of a 

 Polyzoan or Hydrozoan colony, or a proglottis from a 

 tapeworm, may become detached, when it will continue 

 to live and reproduce and exhibit all the character- 

 istic functioning of the species to which it belonged. 



Such an animal as a Hydra, or a Planarian or Chae- 

 topod worm, or a starfish, may be cut into several 

 pieces, and provided that each of these pieces exceeds 

 a certain minimum of mass, it will regenerate the whole 

 structure of the organism of which it formed a part. 

 In the developing embryo of the Sea-urchin the eight- 

 cell stage may be treated so that the blastomeres may 

 come apart from each other : each of them will then 



