378 R. T. GUNTHER. 



In the absence of complicated copulatory organs the 

 Chgetognatha resemble the Amphineura. 



The spermatozoa are pin shaped, as in many Mollusca. 



When we come to compare the arrangement of body 

 cavities, genital organs, and their ducts in the Chaato- 

 gnatha Avith the various arrangements of these organs among 

 the Mollusca, we find that the same structural plan is 

 common to both groups, and that in the Chaetognatha certain 

 primitive features are retained (text-fig. 10). 



The ancestor common to both Chfetognatha and Mollusca 

 had, we imagine, paired but unsegmented coelomic spaces. 

 Gonad mother-cells developed in each half, and were passed 

 to the exterior through paired coelomoducts, which also 

 served for the discharge of renal products. This ancestor 

 we believe to have been hermaphrodite (text-fig. 10, a). 



In the Aplacophoran Proneomenia, Chsetognatha, and 

 Cephalopoda certain of these primitive conditions persist, 

 but not all in the same form. 



In Proneomenia the coelom has become restricted to a 

 relatively small portion of the whole body. The anterior 

 ends of the paired coelomic spaces remain separate as 

 gonocoels; the posterior portions have become confluent, 

 forming the pericardium, which communicates with the 

 exterior by right and left ccelomoducts, which serve the 

 double purpose of genital and renal ducts (text-fig. 10, c). 



In Cephalopoda the posterior portion of the ccelom is 

 the gonocoel; it is undivided, and communicates with the 

 exterior by a pair of gonad ducts (d). The anterior portion of 

 the coelom is paired in Octopotla, and forms the two pericar- 

 dial spaces, each of which has its own duct to the exterior (e). 



In Chiton the gonadial coelom has become separated from 

 the posterior pericardial ccelom, and each has its own pair of 

 ducts to the exterior (f). 



The universal occurrence of paired ducts in these primitive 

 types is, we think, explained by the theory that in the 

 common ancestor the coelomic spaces were divided by a 

 median longitudinal partition. 



