Thomas Huxley, Esq., on Animal Individuality. 175 



Pluteus as a butterfly to its caterpillar ; in the course of de- 

 velopment only a slight ecdysis takes place, the skin of the 

 Pluteus becoming for the most part converted into the skin 

 of the Echinus. 



But in Asterias, the Bipinnaria, which corresponds with 

 the Pluteus, gives up only a portion of its integument to the 

 developed Asterias; the remaining and far larger portion 

 lives for a time after its separation as an independent form. 



The Bipinnaria and the Starfish, are as much forms of the 

 same individual as are the Pluteus and Echinus or the cater- 

 pillar and butterfly ; but here the development of one form 

 is not necessarily followed by the destruction of the other, 

 and the individual is, for a time at any rate, represented by 

 two co-existing forms. 



This temporary co-existence of two forms of the individual 

 might become permanent if the Asterias, instead of carrying 

 off the intestinal canal of the Bipinnaria, developed one of 

 its own ; and this is exactly what takes place in the Gyro- 

 dactylus, whose singular development has been described by 

 Von Siebold. 



But the case of the Gyrodactylus affords us an easy transi- 

 tion to that of the Trematoda, the Aphides, and the Salpae, 

 in which the mutual independence of the forms of the indivi- 

 dual is carried to its greatest extent ; so that even on anato- 

 mical grounds it is demonstrable that the difference between 

 the so-called " skin" of the caterpillar, the free Bipinnaria, 

 and the Salpa democratica is not in kind, but merely in 

 degree. 



Each represents a form of the individual ; the amount of 

 independent existence of which a form is capable, cannot 

 affect its homology as such. 



The Lecturer then proceeded to point out that the doctrine 

 of the " Alternation of Generations" and all theories con- 

 nected with it, rest upon the tacit or avowed assumption 

 " that whatever animal form has an independent existence 

 is an individual animal," a doctrine which he endeavoured to 

 shew, must, if carried out, inevitably lead to absurdities and 

 contradictions, as indeed Dr Carpenter has already pointed 

 out. 



