GENERAL CONSIDERATIONS. 255 



and Plate are not homologous to the branchiae of the Chaetodermatidae, which 

 though a coenogenetic development have nevertheless had an independent 

 origin. Nierstrasz likewise considers the neomenian branchiae to be anal folds, 

 but holds that the Chaetoderma type of gill is the most extreme development 

 of such lamellae. In the Pacific report I have argued in favor of the complete 

 lack of homology between the neomenian type of gill and that found in members 

 of the family Chaetodermatidae. Concerning the true significance of the first 

 named I have no evidence to offer. Plate and Nierstrasz have described cer- 

 tain species {Notomenia clavigera, Archaeomenia prisca, Proneomenia discoveryi) 

 in which the coelomoducts do not open into the branchial cavity, and for the 

 present at least I am not inclined to argue for or against the theory that the 

 neomenian gill is a derivative of the anal wall. But when it comes to the Chae- 

 toderma type of gill the evidence that it is a development of the anal or procto- 

 dael walls is far from being conclusive. If such a type of gill were to be found 

 among the Gastropoda I venture to say it would unhesitatingly be considered 

 as a ctenidium. It has the same macroscopic and microscopic structure, the 

 same blood circulation, musculature, and innervation as the Chiton or Haliotis 

 gill for example, and the space in which it is held contains the outlets of the coelo- 

 moducts and digestive tract. So far as appearance and general relations are 

 concerned the gills of the Chaetodermatidae are true ctenidia, and the sur- 

 rounding space is a mantle cavity. Here again we must have embryological 

 evidence to definitely settle the question. 



Regarding the nature of the ventral fold there is little to add to the observa- 

 tions of other authors and the comments made in the Pacific report. A detailed 

 study of the most advanced larvae of Halomenia gravida, in which the anterior 

 pedal gland comprises three or four cells, shows no line of demarcation between 

 the cells of the mid- ventral line and those more laterally placed; and even if 

 all stages in the development of this organ were present it is doubtful if it would 

 afford convincing evidence that the ventral fold is a foot with a long ancestral 

 history to those opposed to such a view. Thiele, Plate, and Nierstrasz admit 

 that the fold is an organ concerned in the function of locomotion, but that it, 

 with the anterior and posterior pedal glands, is the homologue of the foot of 

 other molluscs is vigorously denied. To them the organ in question has had 

 an independent origin, and where the furrow stops short of the branchial chamber 

 we have a primitive state of affairs. As a matter of fact the groove passes 

 into the branchial chamber in the larger number of neomenians I have studied, 

 and has led me to suspect that at least a portion of this last named space may 



