MR. HUXLEY ON THE ANATOMY OF SALPA AND PYROSOMA. 
579 
homologically, than a highly individualized generative organ, is developed from the 
larva, ova 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 individuality is very 
different from metaphysical individuality, and that the whole question of the pro- 
priety 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 
processes in the higher and lower animals, and the alternation theory becomes un- 
necessary. 
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 Saiga B and Salpa A are only parts of individuals, — are organs ; 
but as we are unaccustomed to associate so much independence and completeness 
of organization with a mere organ, to give them such a name would sound para- 
doxical. It is proposed therefore to call them, and all pseudo-individual form resem- 
bling 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 purely conventional, and established 
for the sake of convenience merely*. 
40. In the Salpce, then, the parent and the offspring are not dissimilar, but the 
individual is composed of two zooids. 
In Cyanea, the individual is composed of two “ zooids,” a medusiform and a poly- 
piform zooid. 
In the Trematoda there are frequently three “zooid” forms to the individual. 
In the Aphidse the sura of from nine to eleven “zooids” composes the individual, 
the great number of zooid forms in this case being simply an instance of that “irre- 
lative repetition” of parts so common among the lower animals. 
A similar irrelative repetition exists among the so-called “compound” animals, 
the Polypes and compound Ascidians ; and consistently with the present theory we 
must call a Sertularia or a Pyrosoma, for instance, not an aggregation of individuals 
* For a further consideration of this subject the author begs to refer to Dr. Carpenter’s “ Principles of 
Physiology,” 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 textto the metamorphosis of the Echinoderms in a Report by himself. — Annals, 
July 1851. 
