76 Nils Hj. Odhner. 
own (cardocoel) independent of other lacunae and coeloms in the 
mesenchyma; this condition is persistent in the Scaphopoda. 
If the genital ducts run in the upper side of the body, immedia- 
tely below the dorsal vessels, the consequence is that the mesen- 
chyma contributing to the construction of the heart is thrown from 
a point of the median side of these ducts. Thus in this case the 
origin of the heart is combined with a fusion at one point of the 
gonoducts: a gonocardocoel rises (Chaetoderma, Neomeniina and 
Mollusca in general). 
If the dorsal vessel runs immediately above the intestine and 
the genital ducts run at its side or below it, the parenchyma sur- 
rounding the intestine will be used to constitute the contractile heart. 
In this case the pericardium becomes inferior to the intestine and 
the latter penetrates the heart. The pericardium may be a specific 
cardocoel or a partial fusion of the gonocoel with the cardocoel. 
(Lamellibranchia). 
In this way we are able to explain the pericardium of the 
Solenogastres as well as that of the Mollusca as a primitive acqui- 
sition. This characteristic feature of the said groups is therefore 
not a result of a regenerative development from a form with a well 
differentiated coelom (Annelids), but shows that we are warranted 
in maintaining a progressive evolution of the two groups in diffe- 
rent directions starting from the parenchymatic worms. From the 
latter they have arisen by acquiring a primary coelom (haemolympho- 
coel) as well as a secondary one (cardocoel); on the other hand 
the Solenogastres remain primitive in lacking the special excretory 
organs which are characteristic of the Plathelminthes. 
The lymphocoel and the haemocoel have not been distinctly 
separated from each other in the Solenogastres, though an incipient 
separation is indicated by the presence of the horizontal septum of 
the ventral vessels (Chaetoderma and many Neomeniina). 
We have in the preceding pages, in accordance with some other 
authors, found reason to assume that the Solenogastres are primitive 
with respect to the Mollusca. Further we have seen that they have 
many characteristics common with the Scolecida and that they 
might be, in certain respects, considered primitive also in comparison 
to the chief representatives of those animals (for example in the 
absence of the protonephridia and of the proboscis of the Nemertines, 
the simpler structure of the genital organs than in the Turbellaria). 
As they, besides these facts, show a certain similarity with Nematods 
