CASES SIMULATING ALTERNATION OF GENERATIONS. 227 



the parent and the embryo, cannot be considered as pro- 

 perly constituting a link of the series. However elaborate 

 their organization, they are not really zooids, either proto- 

 morphic or gamomorphic, for they originate neither the 

 embryos, nor the sexual elements of the embryonic exist- 

 ence ; they serve merely to give a temporary protection to 

 germs which are really of a date of formation anterior to 

 their own. As the gamomorphic zooids have been com- 

 pared to free spermaries and ovaries, so these receptacles 

 may, according to the nature of their contents, be compared 

 to spermathecse, such as occur in insects, only free, or to 

 free uteri, with broods of embryos enclosed their office 

 being not generation but gestation. 



They do, however, simulate some of the more rudimen- 

 tary zooids, both germinal and sexual, so that, under cer- 

 tain circumstances, they may be confounded, if the deve- 

 lopment be overlooked. Thus in the case of Planar ia it 

 appears still doubtful whether the bodies from which the 

 broods of young issue are merely nidimentary envelopes of 

 true ova, or germinal bodies, themselves derived from ova, 

 and originating a fresh progeny by a proper act of gem- 

 mation.* 



Many zooids, indeed, of both orders are known to na- 

 turalists, which, in a structural point of view, are little 

 more than mere egg-sacs, and which in fact are, in some 

 works, designated by this or an equivalent term, such as 

 sporocysts, sporophores, sporosacs, &c. Such expressions, 

 however, are, in strictness, open to objection, being in- 



* Siebold's Comparative Anatomy, Ch. IX., 129, with Dr. W. Bur- 

 nett's note. 



As Dr. Carpenter remarks, in discussing the embryogeny of Purpura, 

 the development of Planaria may prove to be parallel to that of the 

 Pectinibranchiate Mollusca, [noticed in Chap. III.] Trans. Micros. 

 Soc., III., 17. See also Siebold, 293, note 20, on the simulation of 

 the parent cells of spermatozoa, by the spermatophores of some Crustacea. 



