18 HISTORICAL REVIEW. 



theory though he did claim with von Ihering (a theory abandoned by him in 

 '90) that the Amphineura are Vermes. Grobben ('94) Hkewise considered this 

 the correct view though he beUeved the Amphineura to be true molluscs. This 

 notion is implied in the work of Haller ('82), who made the claim that the Chitons 

 and the Solenogastres are distinct groups of animals which have been derived 

 from a common vermian ancestor. In a more vigorous fashion Thiele argues 

 from the same standpoint. 



With one or two exceptions those who argue along the line just indicated 

 regard the Solenogastres as primitive animals, and are accordingly opposed to 

 several investigators who hold a diametrically opposite view. Simroth, Wiren, 

 and Heath believe that the Solenogastres early branched off from some primitive 

 polyplacophore and while retaining several primitive features are in other re- 

 spects degraded organisms. Pelseneer and Garstang take practically the same 

 view. Marion, in a sense, does the same as he compares the adult Solenogastre 

 to the larva of the Chiton. Plate traces the Solenogastres and Chiton lines of 

 descent to some ancestral mollusc which may have given rise also to the present 

 classes. 



In regard to the derivation of the molluscs, and the Solenogastres especially, 

 from some premolluscan ancestor there are a number of widely divergent theories. 

 In 1877 von Ihering believed that among the worms the gephyreans are most 

 closely related to the Solenogastres. Haller ('82) on the other hand regarded 

 them as more closelj' allied with the nemerteans. Hubrecht, Thiele, Plate, and 

 a number of other writers consider that the molluscs, or at all events the Soleno- 

 gastres, arose from a turbellarian-like ancestor. This idea has been most fully 

 developed bj' Thiele. According to him the progenitor of the molluscs and the 

 Solenogastres (which are considered to be worms) was in the fundamental 

 characters of its organization similar to the modern cotylean poly clad. The 

 often frilled sensory margin of the body became the mantle, which for purposes 

 of protection, developed a cuticular covering and ultimately a shell, while the 

 ventral sucking disc expanded into the molluscan foot which in its least modified 

 form occurs in Haliotis and similar species. 



