74 Nils Hj. Odhner. 
primitive organization in the slighter development of the lympho- 
coel, a feature which more approaches them to the Nemertines and 
the Turbellarians and which depends on the large dimensions of 
the intestine, the diverticula of which in a high degree replace a 
system of lymphatic lacunae. 
It is true that Chaetoderma in some respects recalls a Chiton, 
so that a genetic relation between the two is suggested. The closest 
agreement is manifested rather in the similar development of ihe 
haemo-lymphatic (primary) coelom in both forms. But Chaeto- 
derma represents the most primitive condition, seeing that here 
the blood vessels (haemocoel) are still to a great extent contained 
-in this coelom, whereas in Chiton they are differentiated in a rather 
high degree. Even the remaining part of the primary coelom shows 
in Chiton a higher stage of differentiation in the presence (at least 
in certain forms) of a continuous endothelium, a secondary appear; 
ance, which has given rise to the interpretation of this coelom as a 
part of the secondary body cavity, though evidently, with regard 
to its function and as compared with that of Chaetoderma, it must be 
considered as the primary coelom, the lining cells of which have 
been secondarily joined to an endothelium. 
For these reasons, taken together with the fact that Chaeto- 
derma shows agreements with the Nematods one must conclude 
it to be more likely that the Chitons have developed from a form 
similar to Chaetoderma than vice versa. The Chiton has a foot 
homologous with the whole ventral side of Chaetoderma and 
Neomenia as well as a mantle furrow between this foot and the 
notum. The homologon of this furrow is, probably as a consequence 
of the well developed tegmentum (cf. Gymnomenia) absent in Chae- 
toderma and Neomeniina. The cloaca, which has been considered 
as a sort of mantle cavity, is certainly another kind of invaginatory 
formations, probably comparable with the cloaca in the Nematods, 
though generally furnished with respiratory organs as secondary 
acquisitions. The possibility may however be admitted that this 
anal room, or cloacal chamber, however it may be called, may be 
absorbed into the mantle cavity when septal muscles give rise to 
one in connection with the differentation of the foot. Further the 
Chiton has acquired a nephridium probably homologous to the — 
coelomoducts of the Solenogastres, whereas its genital ducts seem 
to have originated secondarily. PLATE (1901, p. 496)) has esta- 
blished some facts on the development of the genital ducts in young 
