GENERAL CONSIDERATIONS. 253 



Chactoderma altenatum, and again it may be entered bj^ two vessels but this 

 bipartite condition ends at the pericardial wall. The impression given is that 

 the heart is a relatively simple tube, usually ' with two divisions, sometimes 

 sacculated, but I ha^•e never found more than one auriculo-ventricular opening 

 or any other e\idence to show that the heart is a paired organ. 



Whether the auricle or atrium is the homologue of the auricles in other 

 species of molluscs is Ukewise uncertain ; there is no clear evidence for or against 

 such a view. And we are, it seems to me, equally in the dark when we approach 

 the subject of the most primitive type of Solenogastre heart. In my opinion 

 the heart wliich lies in a fold of the pericardial wall as in some of the Proneo- 

 meniidae appears to be among the most primitive. On the other hand where it 

 lies freelj^ in the pericardial cavity, as in Chaetoderma or Alexandromenia, it 

 must have arisen from a simpler embryonic condition, and a simpler phjdogene- 

 tic stage is equally conceivable. Here again the matter rests upon few data 

 and personal opinion and must accordingly remain as an unsettled problem 

 for the present. 



The digestive system, Uke the circulatory and muscular systems, is most 

 susceptible of change, and the wide variations of form and component elements, 

 correlated with differences in habits of life of the different species, renders it 

 difficult to differentiate coenogenetic from palingenetic characters. I believe, 

 however, that in the ancestral Solenogastre the fore gut was provided \\'ith both 

 dorsal and ventral saUvary glands and a radula, while the mid-gut, as in the 

 modern neomenians, was a relatively simple tube without clearly defined stomach, 

 digestive gland, and intestine. The hind gut appears to me to be a relatively 

 small section of the digestive tract in the Chaetodermatidae, and forms no 

 part of what has been termed the cloacal, anal, or mantle chamber, a point 

 to which I shall return. As Thiele maintains the atrium of the Neomeniina 

 is no part of the fore gut, and it is possible that it is the homologue of the buccal 

 shield in the Chaetodermatina and the snout of the Chitons. 



Dorsal and ventral saUvary glands clearly appear in several species, such 

 for example as Proneomenia hawaiiensis and Lophomenia spiralis. On the other 

 hand the ventral set may disappear completely as in Limifossor and several 

 species of Strophomenia. Since dorsal glands exist in Limifossor I am incKned 

 to look upon the diffuse glands attached to the fore gut of Chaetoderma as a 

 modified homologue. \\Tiether this is the case with the equally diffuse glands 

 surrounding the walls of the fore gut in Strophomenia it is impossible to determine. 

 In Alexandromenia there are three distinct groups of salivary glands one of 



