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ON THE REPRODUCTIVE SYSTEM IN THE HYDROIDA. 423 
species is composed, and a proper conception of its heteromerism is of vital 
importance in any attempt at the determination of affinities and a philoso- 
phical arrangement of species. 
- With scarcely an exception then, so far as we yet know, every hydroid 
consists at one period of its life of an assembly of zooids, and the true zoolo- 
gical individual is in reality the sum of all these zooids, whether they remain 
permanently associated or detach themselves from one another in order to 
become henceforth independent organisms. 
This remarkable phenomenon may be termed the Polymerism of the Indi- 
vidual. But the peculiar characters of the hydroid individual do not stop 
with mere polymerism ; the zooids composing it may, and, so far as our know- 
ledge extends, always do present a greater or less amount of dissimilarity 
among each other, so that the individual is not merely polymere, it is also 
heteromeric ;. and this distinction is of importance to be kept in mind ; if we 
would form an adequate conception of the true hydroid individual *. 
The extent to which heteromerism may be carried varies in different 
species. We may have the total result of the development of a single 
ovum composed of two different forms, or of three, or of four; in other 
words, the zooid elements whose sum constitutes the zoological individual 
may be dimorphic, trimorphic, or tetramorphic, Clava, Coryne, Tubularia,. 
&ec., present examples of dimorphism, the two forms here produced being 
the polypite and the gonophore. Dicoryne and most of the angiogonial 
hydroids afford examples of trimorphism—the gonoblastidium-form being 
in these cases added to those of the polypite and gonophore ; while in Lao- 
medea dichotoma, Campanularia Johnstoni, &c., we have instances of tetra- 
morphism,—for in these hydroids the medusa which buds from the blasto- 
style properly remains sexless, giving rise by gemmation to the true sexual 
gonophore, which is in the form of a sporosac borne as a bud upon its 
radiating canals (see above, p. 401). Hydractinia echinata also affords among 
the gymnogonial hydroids a very interesting example of tetramorphism. In 
this hydroid the colony is composed of four different forms of zooids. 1. The 
ordinary nutritive polypites. 2. The gonoblastidia destined for the support of 
the gonophores: these may be morphologically regarded as polypites arrested 
in their development, the tentaclés having become reduced to the condition 
of mere tubercles loaded with thread-cells, and the mouth remaining proba- 
bly in most cases undeveloped, though it would seem to be occasionally pre- 
sent; they may be easily compared to the blastostyle of the angiogonial 
genera. 3. The gonophores or imperfectly developed and disguised me- 
duse. 4. A set of peculiarly modified polypites, which, like the ordinary 
polypites, properly belong to the trophosome ; the tentacles have become 
aborted in them, being represented only by small hemispherical groups of 
* The expression “heteromerism of the individual” is intended to conyey nearly the 
same idea as Leuckart’s “‘ polymorphism of the individual” (see Leuckart, Ueber den 
Polymorphismus der Individuen, Giessen, 1851). The term “ individual,” however, is 
used by Leuckart for the separate zooids, which are, properly speaking, only the elements 
of which the zoological individual is composed; so that, with our present conception of an 
individual as the logical element of the species, the expression “ polymorphism of the indi- 
vidual” would have a different meaning from that which Leuckart has assigned to it. I 
have therefore, instead of ‘‘ polymorphism of the individual,” used the phrase “ hetero- 
merism (€repos, pépos) of the individual,” which easily conveys the intended meaning ; 
while we may use that of ‘‘ polymorphism of the zooids”’ to express the fact that the zooids 
are not all of the same form. ‘The phrases “ heteromerism of the individual ” and “ poly- 
merism of the individual” may, it is true, be objected to on the grounds that they are self- 
contradictory ; but this is the result of the new ideas which have become involved in our 
conception of the biological individual, which is no longer necessarily ‘‘ individwus,” 
