HISTORICAL REVIEW. 17 



f^roup of animals. The more special features relating intimately to the various 

 systems are noted to some extent in the main body of the present paper, while 

 the general considerations are discussed on p. 1G4-173. Among the more exten- 

 sive of these works are those of Pruvot ('91), who has described several species 

 from the shores of France; Wiren ('92) whose study of several species from the 

 Scandinavian coast is among the best that has ever appeared; Simroth ('93) and 

 Pilsbry ('98) whose systematic works are of the highest value; Thiele whose 

 various papers during the past fifteen years have added materially to our knowl- 

 edge of the anatomy of molluscs, including several species of Solenogastres ; 

 Nierstrasz (1902, etc.), who with an abundance of material collected chiefly by 

 the Siboga Expedition, has added extensively to our information of these animals; 

 and finally the present writer who has contributed some data relating especiallj' 

 to the nervous system. 



Since the above was written Nierstrasz has published an important report 

 (1908) reviewing the work of various investigators since the appearance of Sim- 

 roth's paper in 1893. It is a valuable contribution, and the scheme of classifica- 

 tion there adopted will be of much service. 



Turning now to the broader features of the classification of these animals 

 we find that practically every investigator in this field of research is agreed with 

 Spengel that among the Mollusca their nearest relatives are the Chitons; but 

 regarding their more accurate position within the phylum differences of opinion 

 appear. Von Ihering in 1877 called attention to the fact that the Solenogastres 

 and the Chitons are not distantly related, especially if we consider Chitonellus 

 to be a connecting link and therefore a primitive animal. According to this 

 line of reasoning the Solenogastres, devoid of a radula (none had been discovered 

 at that time) and shell, are the more ancestral and are so closely related to the 

 worms that both they and the Chitons constitute a special phylum (Amphineura) 

 of Vermes as noted in a previous paragraph. Hubrecht, without laying much 

 stress on the ancestry of the Amphineura, though he hints at their derivation 

 from a platyhelminth ancestor, was likewise of the opinion that the Soleno- 

 gastres are primitive, and that Chitonellus is a link connecting them with the 

 more highlj' modified Chitons. It may be mentioned also that Haller in '94 

 modified some of his previous ideas, having become convinced of the correctness 

 of von Ihering's and Hubrecht's position. 



The above idea was combated by Pelseneer ('90) who claimed that Chito- 

 nellus is nothing more than a highly modified Chiton and in no direct way related 

 to the modified group of the Solenogastres. Hatschek ('91) agreed with this 



