GENERAL CONSIDERATIONS. 169 



of the heart the aorta arises and passing along the dorsal side of the gonad, that 

 it supplies in Chiton-like fashion, it reaches the head cavity. In the great 

 majoritj' of species this last named organ is not clearly defined, liut in Limifossor 

 it is separated from the visceral cavity by a connective-tissue septum as clearly 

 defined as in the Chitons and having essentially the same relations. Within 

 the head sinus the blood makes its way by irregular channels into the visceral 

 cavity and passes backward. In Limifossor the septum is perforated ventral 

 to the intestine and through this the blood makes its way into the ventral sinus. 

 A special visceral artery or sinus is lacking within the group, its function being 

 taken by the general visceral cavity and ventral sinus. This last named space 

 communicates freely at many points with the visceral cavity and posteiiorly 

 both unite and the combined vessel makes its way to the ctenidia and from 

 thence into the heart. 



Hansen years ago noted the presence of crystals in the coelomotlucts of 

 Chaetoderma, and considered it possible that they may function as kidneys. 

 From much more extensive studies Wiren has taken the same position, showing 

 the close similarity of the tissue to that of the Chiton kidney. Another fact of 

 the greatest importance is that in Chaetoderma erudila for example, where the 

 sexes are separate the tiephridia of the male are exactly the same as those of the 

 female. If the coelomoducts here act in the capacity of shell, mucous or other 

 glands intimately connected with the egg-laying process it is reasonable to 

 suppose that it would be more highly developed in the female than in the male. 

 Since it is not it becomes much more probable that the resemblance of the cells 

 of the duct to those of the Chiton kidney is not accidental, but that they are 

 true excretory elements and the ducts therefore have retained the excretory 

 function derived from the ancestral form. 



Stating the matter in another way it appears that the coelomoducts are, 

 from the standpoint of both structure and function, of a more primitive character 

 in the Chaetodermatina than in the Neomeniina. In the latter family the ducts 

 have assumed an important role in the storage of sperms, or in the development 

 of envelopes for ova and perhaps other processes connected with egg-laying, 

 so that the original function of excretion is effectually masked, if it exist at all. 

 In the Chitons the kidneys become active excretory organs long before any trace 

 of the gonad or its ducts appear, and if these tubes in the Neomeniina act as 

 kidneys they likewise would probably assume their duties at an early stage. 

 In Dorymenia acuta, however, there are no signs of such activity in individuals 

 14 mm. in length. The reproductive glands are present, though in a very im- 



