14 HISTORICAL RKVIEW. 



the group. At this time JSpengcl describod tlie innervation of the osphradiiun, 

 and in several species of molhiscs discovered deep-seated resemblances in the 

 elements of the nervous system and tlieir arriuigcnu'nt. (_)n the l)asis of this 

 work, which includes the examination of Neomenia, C'haetoderma, and two 

 unidentified species of Solenogastres, antl with the additional helj) alfoi'ded l)y 

 the work of Tullberg, von Graff, and Hansen he emphatically claimed, in ojipo- 

 sition to von Ihering and Gegenbaur, that the Amphineura are true molluscs. 

 Accordingly he established the Amphineura (Chitons and Solenogasti'es) as a 

 class of the MoUusca. 



A very short time afterward the masterly work of Hulirecht appearetl, 

 and in some respects it continues to be the most important work that has ever 

 been published on the subject. The study was based chiefly upon a gigantic 

 species, ProneoDioiia sluidri, which occurs in Barents Sea north of Scandinavia, 

 and embraced a careful examination of its external and internal anatomy. The 

 results, with a few relatively imimiiortant exceptions, have been confirmed by 

 the study of many other species, and form a most substantial foundation for 

 studies of more i-ecent date. Concerning tlie relationships of Proneomenia, 

 Neomenia, and Chaetoderma the author has no hesitancy in agreeing with 

 Spengel that they constitute one order (Solenogastres) of the class Amphineura, 

 the Chitons belonging to the other (Polyplacophora). In a number of suc- 

 ceeding publications this position is held without modification, and the few 

 additional facts of importance that are presented still further emphasize the 

 correctness of the theory. 



From this time forth scarcely a year has elapsed withmit one or more papers 

 appearing on the subject of the Solenogastres. Deep-sea researches or work 

 along the shore line beyond the littoral zone have brought to light an ever 

 increasing number of species whose anatomy is now for the most part fairly 

 well known. Without exception all are built upon essentially the same funda- 

 mental plan though in detail each species presents, as is to be expected, some 

 new and interesting modifications. To the majority of zoologists the accumu- 

 lated results point unmistakably to the true molluscan nature of these animals, 

 but a glance through some of the succeeding paragraphs will show that there 

 is far from being a unanimity of opinion regarding their position in the phylum 

 and their relation to other groups. The great mass of anatomical details which 

 have been published during the past twenty-five years, serving chiefly to dis- 

 tinguish genera and species, new and interesting though they may be, can be 

 but briefly considered in a review of this character. 



