52 ANATOMY AND PHYSIOLOGY OF SALPA AND PYROSOMA 



has done (70), the larva never produces the imago by sexual genera- 

 tion, the imago again producing the lar\-a by sexual generation. But 

 a pseud-imago, which is indeed nothing more, homologicall}', than a 

 highly individualized generative organ, is de\-eloped from the lar\-a, o\'a 

 are produced by it, and from these the larva again is developed ; the 

 whole process differing from that common to animals in general, in 

 nothing but the independence and apparent individuality of the- 

 generative organ. 



39. It cannot be too carefully borne in mind that zoological indi- 

 \-idualit}' is verj- different from metaphysical individuality, and that 

 the whole question of the propriety of the " alternation theory " as a 

 means of colligating the facts (for at best it can be nothing more), 

 turns upon the nature and amount of this difference. 



If the true definition of the zoological individual be (as the writer 

 believes it to be) " the sum of the phenomena successively manifested 

 by, and proceeding from, a single ovum, whether these phenomena be 

 invariably collocated in one point of space or distributed over many," 

 then there is no essential difference between the reproductive pro- 

 cesses in the higher and lower animals, and the alternation theory 

 becomes unnecessar}-. 



In accordance with this definition, neither the form A, nor the 

 form B would be a zoological individual ; not either of their forms, 

 but both together, answer to the " individual " among the higher 

 animals. 



In strictness both Salpa B and Salpa A are only parts of individuals,, 

 — are organs ; but as we are unaccustomed to associate so much inde- 

 pendence and completeness of organization with a mere organ, to give 

 them such a name would sound paradoxical. It is proposed, there- 

 fore, to call them, and all pseudo-individual forms resembling them,, 

 " zooids," bearing in mind always that while the distinction between 

 zooid and individual is real, and founded upon the surest zoological 

 basis, — a fact of development, — that between zooid and organ is 

 purel}' conventional, and established for the sake of convenience 

 merely.^ 



40. In the Salpce, then, the parent and the offspring are not dis- 

 similar, but the individual is composed of two zooids. 



In Cyanea, the individual is composed of two " zooids," a medusiform 

 and a polypiform zooid. 



^ For a fiiithei- consideration of this subject the author begs to refer to Dr. Carpenter's 

 "Principles of Phj'siology," in which the whole question of individuality in plants and 

 animals is treated in a very clear and masterly manner ; to Mr. Thwaites's papers in the 

 Annals of Natural History ; and to an attempt to apply the principles advocated in the text 

 to the metamorphosis of the Echinoderms in a Report by \ma%t\[.~Annah, July 1851. 



