140 THE NATURE AND VARIETIES OF 



rence and in the gamomorpliic stage, that it appears as a 

 case of distinct alternation, because in the two former the 

 gemmae remain in union mth the parent stock, instead of 

 assuming the character of free zooids. 



The remarks now made, and those which the course of 

 the argument will yet call for, touch, I think, upon all the 

 cases of alternation in the Animal Kingdom, above referred 

 to, with the exception of the Salpce, to which, therefore, a 

 passing reference may here be made. 



§ 9. Notwithstanding that the reproduction of the Salpoe 

 was the first case of " alternation" to attract notice, and 

 that it has ever since been res-arded as among; the most 

 striking instances of it, it is at the same time one concern- 

 ing the exact relations of which some doubt may be enter- 

 tained. From the associated forms having, along with 

 perceptible differences in external configuration, such a close 

 similarity in internal structure, and from their locomotive, 

 circulatoiy, digestive, and nervous systems being so entirely 

 on a par in the scale of organization, it would be hazardous 

 to pronounce one more than the other the typical represen- 

 tative of the species, so that we are in a sort of difficulty to 

 decide whether the alternation is due to the interpolation 

 of a protomorphic zooid — the solitary asexual form which 

 originates directly from the ovum — or of a gamomorphic 

 zooid — the catenated sexual form budded off from the 

 former. In point of organization there is nothing to indi- 

 cate to us whether the gemmation precedes or follows the 

 orthomorphic stage of development. From all that we can 

 gather from a comparison of the two forms, we might either 

 compare the solitary Salpa to the gregariniform phase of 

 Trematode life, and the catenated Salpcc derived from it to 

 the brood of Distomas — both alike maturing their reproduc- 

 tive structures as simple organs in the continuity of their 

 own bodies — or, on the other hand, we might consider the 

 solitary form as the typical one— elaborated out of the 



