245 



directed upward since the animal lies upon the side opposite 

 the siphons. In 17 out of 19 cases the animal was found to lie 

 upon its left side. 



Another difference between M. manhattensis and M. rohusta is 

 shown by the color of the eggs and ovaries. In the former 

 species they are colorless, while in the latter they are reddish 

 purple. After the eggs of M. rohusta are shed, the ovaries lose 

 much of their color. 



Still another difference is evident in the character of the tunic, 

 that of M. rohusta being relatively thick as well as entirely opaque, 

 and when viewed from the inside it has a purple color. The tunic 

 of M. manhattensis is slightly translucent and is colorless. 



According to the observations here presented, M. rohusta differs 

 sufficiently from M. manhattensis in embryology, morphology, 

 and ecology, to justify the classification of the two forms as 

 distinct species. 



The classification of animals in the past has usually been based 

 upon adult structures, but that such a basis is inadequate is 

 evidenced by the uncertainty in the classification of these two 

 species of Molgula. It is necessary, therefore, that beside the 

 adult form, the complete life history also be considered, and 

 particularly is this true in the ascidians. Here we often find that 

 the factors of evolution have caused a great diversity in the 

 embryology of different species, though the adults may be very 

 similiar. 



Summary. 



1. M. manhattensis has a larva with a free-swimming period, 

 while M. rohusta has not. A statolith is present during this 

 period in the former and absent in the latter. 



2. M. manhattensis is found attached, while M. rohusta is 

 unattached. 



3. M. manhattensis is symmetrical, while M. rohusta is asym- 

 metrical and tends to lie upon its left side. 



4. The eggs and ovaries of M. manhattensis are colorless, while 

 those of M. rohusta are reddish purple. 



5. M. manhattensis has a relatively thin, translucent tunic, 

 while that of M. rohusta is relatively thick and opaque. 



LITERATURE CITED. 

 De Kay, J. E. 



1843. Zoology of New- York, part 5, Mollusca. Albany: vii+27l!p., 40pl. 

 Grave, Caswell. 



1926. Molgula citrina (Alder and Hancock). Activities and structure of 



