146 



THE AMERICAN NATURALIST 



[Vol. LIII 



parent, however, that the matter needs considerable 

 further investigation before this question can be definitely 

 determined. 



The affinities of the molluscan group are somewhat 

 obscure, but the study of their immature stages would 

 indicate that the Mollusca are rather distantly related 

 to the Chaetopoda, Gephyrea, "Polyzoa," and other 

 annelid-like forms. If this be true, their line of develop- 

 ment apparently likewise extends back to forebears simi- 

 lar to the members of the Rotifera-Platyhelminthes 

 group (which are very like the ancestors of the ''Anne- 

 lida" also), and the ancestors of the Ecliinodermata and 

 Hemichordata may possibly be traced back to a similar 

 stock (or to forms closely related to them) as well. 



The Hemichordata are regarded by many authorities 

 as a group which has departed but little structurally 

 from the ancestral condition of the forms leading up to 

 the vertebrate type of animal. From a study of their 

 larvre, some investigators have concluded that the Hemi- 

 chordata are related to the Echinodermata ; but this 

 brings us no nearer to the solution of the problem, since 

 the Echinodermata occupy an isolated position, and their 

 ultimate affinities are very obscure. Although the Echi- 

 noderm line of development may lead back more directly 

 to the Coelenterata, there is a possibility of their fore- 

 bears being quite closely related to the members of the 

 Rotifera-Platyhelminthes group wliich have departed 

 but little from the condition characteristic of the ances- 

 tors of the ' ' Annelida ' ' and other invertebrate groups ; 

 and even if the Echinodermata are to be regarded as the 

 descendants of Ccelenterata-like forebears, it must be re- 

 membered that the ancestors of the Eotifera-Platyhel- 

 minthes group were themselves very closely related to 

 the Coelenterata, and w^ould probably have been classed 

 as such, rather than as colonial Protozoa. 



It is quite possible to regard the line of development 

 of the Echinodermata as related to the other two lines of 

 development in question, and as branching otf near, or 



