IGS GHNEKAl- COXSl DERATIONS. 



licfiardins thi' inaiitlc cavity I Ik'Ucvc tliat it as truly exists in the Soleno- 

 pistres as in tlic Cliitons or prosoliranchs, for example. The sole reason for 

 considcriufi that the so-called branchial or cloacal cavity is a secondary modifi- 

 cation apiK-ars to have chiefly originated with Thiele who claims that the bran- 

 chial lamellae of various neomenians are highly developed rectal folds and 

 accordingly the branchial cavity is nothing more than a greatly expanded rectum. 

 The same is true of the Chactodermatidae, for the plume-like respiratory organs 

 are said to have been developed from similar rectal folds, and accordingly their 

 remarkable resemblance to true ctenidia is of no especial significance. Further- 

 more the fact that the nephridia open into this space is likewise of no importance 

 for it is of coenogenetic origin. 



Regarding the branchial plates of the Neomeniina, they certainly have every 

 appearance of being merely folds of the walls of the branchial cavity, but that 

 they are closely related phylogenetically to the respiratory organs of the Chaeto- 

 dermatidae is an entirely different matter and one most difficult to substantiate. 

 On the other hand it seems to me that the gills of this last named family are not 

 clearly homologous with the neomenian resjiiratory organs. In a former pa])er 

 I have called attention to the fact that in gross and microscopic appearance, 

 blood supi)ly, and innervation they are practically identical to the Chiton gill. 

 Such an idea brings us without violence to the belief that in the original ancestor 

 of the Solenogastres and Chitons there was a true mantle cavity containing at 

 least two ctenidia, the separate outlets of the urogenital system and possibly 

 an osphradium though such an organ may well have been in a difTuse condition 

 as in some of the modern lamellibranchs. Accordingly the connection between 

 the pedal furrow and the mantle cavity is not secondary but primitive and 

 similar in its broader features to what is found in the Chitons. The polybran- 

 chiate character of this last named group yet remains a puzzle for as I have 

 pointed out ('05) there is nothing in the development of these organs to indicate 

 if it be primitive or not. 



While several fundamental differences between the circulatory systems of 

 the Solenogastres and the Chitons have been found to exist these may be ex- 

 plained to some extent on the supposition that originally the foot was of larger 

 size, and in any event they do not outweigh several remarkable resemblances. 

 In both the pericardium is dorsal, posterior, and communicates with the exterior 

 with paired ducts. In present day species it contains the heart, a simple tubular 

 organ or differentiated into a ventricle and single auricle, which may originally 

 have been paired as Wiren has ingeniously suggested. From the anterior end 



