THE CH^TOGNATHA, OR PRIMITIVE MOLLUSCA. 383 



Wheu, as is typically the case in Mollusca, the right and 

 left coelomic cavities become confluent, one of the original 

 pair of ducts may be dispensed with. Confirmatory evidence 

 for the primitive separation of the coelomic cavities into right 

 and left among Mollusca is to be found in the paired structure 

 of the gonads of Aplacophora and Lamellibranchia, in the 

 paired origin of the pericardial coelom in Paludina, and in 

 the very widely-distributed, paired, renal cavities. In the 

 Chgetognatha the primitive division of coelomic spaces into 

 right and left halves is retained throughout life. 



Thus far our comparison lies on certain ground. The next 

 point to be considered is the significance of the division of 

 the perigonadial coelom by the transverse genital septum in 

 the Chaetognatha, and this is capable of more than one inter- 

 pretation. If we regard this genital septum as a division 

 between two homomeric segments we shall adopt the simplest 

 explanation. The condition of transversely-divided coelomic 

 spaces in Chiton would be evolved from the Chtetoguath con- 

 dition on the assumption that the posterior cavity has lost 

 the power of developing gonads, and that its ducts have 

 become purely renal in function. 



The genital coelom of Chiton would, on this theory, be the 

 homologue of the anterior pair of body cavities of Sagitta. 



Unfortunately the observed facts of development do not 

 support the view of a metameric repetition of these cavities. 

 The genital rudiments in Mollusca first appear as ridges on 

 the pericardial walls, and therefore in a cavity which is not 

 metamerically repeated, and grave doubts have been recently 

 thrown upon the view that the genital septum of Sagitta is 

 an indication of metameric segmentation by the above- 

 mentioned observations of Doncaster. 



We are thus led to the conclusion that Sagitta and primi- 

 tive Mollusca, like the Aplacophora, had but a single pair of 

 gonadial-renal-coelomic cavities, into which the hermaphrodite 

 gonads were dehisced and from which they passed to the 

 exterior by a single pair of ducts, together with the nitro- 

 genous waste. By the differentiation of the walls of this 



