IN THE ANIMAL KINGDOM. 89 



mation becomes still more marked, and gives rise, by the 

 continued pullulation of new zooids, to the formation of the 

 variously branched polypidoms, which are so characteristic 

 a feature of the class, as is indicated by the names which 

 different naturalists have applied to it. The species are 

 perhaps never solitary, but in a few cases they occur in 

 pairs, the gemmation stopping short at the initial stage of 

 the production of a double embryo. 



The peculiarity referred to in the gamomorphic stage of 

 the life history is that the structures elaborating the sexual 

 elements are developed at so late a period, and in a manner 

 so similar to the pullulation characteristic of the class, as to 

 give them less the appearance of mere organs, than of dis- 

 tinct gemmae or attached zooids like the polypes themselves. 

 The fuller discussion of this point, however, must be re- 

 served till the general relations of the organs of reproduc- 

 •tion come under consideration. 



§ 6. REPRODUCTION IN THE TUNICATA. 



The Tunicata, which, along vdth. the Polyzoa, constitute 

 the inferior division of the MoUuscan sub-kingdom, have 

 long been kno^\^l to propagate both by impregnated ova and 

 by gemmae. The latter are generally formed on long 

 tubular processes emitted from the parent stock, which are 

 equivalent to, and sometimes closely resemble, the hollow 

 polypidoms of the Polyzoa. As in that class, too, the gem- 

 mae frequently give rise by their cohesion to compound 

 structures, quite distinguishable, however, by characteristic 

 differences in the connection and disposition of the com- 

 ponent zooids. This arrangement prevails especially in 

 certain families, while solitary forms are more characteristic 

 of others. 



But a difference in this respect is not always to be re- 

 garded as a specific character, for it is well ascertained that 



