IN THE ANIMAL KINGDOM. 91 



new alternation. In the living state, the different modes of 

 connection to the parental system of the embryo and the 

 gemmse are at once indicated by the continuity of the cir- 

 culation in the latter, and its participation in all the oscil- 

 lations and irrea^ularities of that of the adult, while the 

 embryonic current is quite distinct, notwithstanding the 

 vessels are curiously interlaced with those of the parent in 

 the medium of attachment, which has very much the struc- 

 ture of the placenta of a Mammalian. 



Changes of form also of the nature of metamorphosis 

 occur in the development of some Tunicata. Thus the 

 young of Ascidia quit the egg in the form of a Cercaria or 

 microscopic tadpole (Spinula of Dalzell), which is afterwards 

 transformed into that of the adult by the loss of the tail, 

 and the evolution of the characteristic organs from the sub- 

 stance of the head portion. 



§ 7. REPRODUCTION OF THE HIGHER MOLLUSCA. 



Among the Mollusca proper, gemmation is a very excep- 

 tional phenomenon. Not only is alternation unknown at 

 any stage of the life history, but even the implantation of 

 the embryonic structure* on the primitive germinal mass — 

 which is so characteristic a feature in the development of 

 segmented animals — is here replaced by a process of trans- 

 formation of the entire yolk into the substance of the em- 

 bryo, and the origination of all the organs of the latter in 

 the cells that are formed by the sub-division of the former. 



The most remarkable phenomena in Molluscan embryo- 

 geny appear indeed at first sight to indicate a process of 

 exactly the opposite kind — the fusion of numerous germinal 

 masses into a single embryo, Koren and Danielssen have 



* Dr. Carpenter, Principles of Compar. Physiol., 4th Ed,, p. 579. In 

 the highest class — the Cephalopoda — the embryonic structures originate 

 from one point of the vitellus, as in most other ova. 



