338 INVERTEBRATE MORPHOLOGY, 



usually open directly into the suprabrancliial chamber quite 

 near the openings of the nephridia (Fig. 149, go) ; conditions 

 connecting these two extremes are to be found, as in Pecten, 

 where the reproductive ducts communicate with the nephridia 

 near their distal ends, and in Cyclas and Ostrea, where both 

 uephridial and reproductive openings are contained in a 

 common groove. No complex accessory structures are de- 

 veloped in connection with the reproductive organs, as in 

 some of the Gasteropods, nor is there an intromittent organ 

 in the male, the ova and spermatozoa being usually extruded 

 to the exterior, where fertilization takes place, or else the ova 

 pass from the suprabrauchial chamber into the iuterlamellar 

 spaces of the gill-plates and are fertilized there. 



The structure of the gills forms a suitable character for a 

 classification of the Pelecypoda. 



1. Order Protobranchia. 



The gill is a true ctenidium attached by its axis to the 

 roof of the mantle-cavity in its posterior part. In addition to 





f 

 FIG. 151. Nucula nucleus FOR THE LEFT SIDE AFTER THE REMOVAL OF 



THE LEFT SHELL, AND LEFT MANTLE-LOBE (after PELSENEEB). 

 aa = anterior adductor. / = foot. 



ar - anterior retractor pedis. g reproductive organ. 



c = cteuidium. p - labial pulp. 



cp = levator pedis. pa == posterior adductor. 



pr = posterior retractor. 



this primitive feature the foot has a creeping surface, the 

 pleural ganglia are not completely united with the cerebral, 



