18 HISTORICAL REVIEW. 



theory thougli ho did claim with von Ilierins (a theory abandoned hy him in 

 '90) that tlie Am])hineura are Vermes. (Irobl^en ('94) hkewise considered this 

 the correct vicnv tliougli he beheved 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 Soleiiogastres 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 jirimitive features are in other re- 

 spects degraded organisms. Pelseneer and Ciarstang take jiractically 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 b(>lieved that among the worms the gephyreans ai'e most 

 closely related to the Solenogastres. Haller ('82) on the other hand i-egarded 

 them as more closely 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 by Thiele. According to him the progenitor of the molluscs and the 

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

 characters of its organization similar to the modern cotylean polyclad. The 

 often frilled sensory margin of the body became the mantle, which for jiui'poses 

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

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

 form occurs in Haliotis and similar species. 



