436 THE ANATOMY OF INVERTEBRATED ANIMALS. 



backward, so that nearly-related forms are sometimes opistho- 

 branchiate, sometimes prosobranchiate. The branches of the 

 aortic trunk soon terminate in lacunae, by which the blood is 

 conveyed back to the walls of the mantle-cavity. The renal 

 organ is a contractile sac with delicate walls, which opens on 

 one side into the pallial chamber, and on the other into the 

 pericardial sinus. 



The Thecosomata have the principal ganglia concentrated 

 around the gullet— the cerebral ganglia being lateral, and 

 united by a long commissure. 



In the Gymnosomata the ganglia are more scattered, but 

 the arrangement of their nervous system needs reexamina- 

 tion. m . 



All the Pleropoda are provided with an ovotestis. This 

 is a racemose gland, in the ultimate caeca of which both ova 

 and spermatozoa are developed. The spermatozoa make 

 their appearance at the closed end of the caecum and accumu- 

 late in its cavity ; the ova are developed from the epithelial 

 tissue of the caecum, somewhat lower down; nevertheless 

 fecundation does not take place in the ovotestis, probably in 

 consequence of the ova and spermatozoa attaining maturity 

 at different times. The ovotestis has a single excretory duct, 

 the termination of which may be provided with a reeeptaculum 

 seminis and connected with a penis. 



The young of the Pleropoda leave the egg provided with 

 a velum, with a rudimentary shell, and probably with an 

 operculum. In most of the Thecosomata the shell is re- 

 tained and forms the commencement of that of the adult, 

 while the vela disappear and the epipodia are developed. 

 In Cymbulia, the primary external shell is shed and the 

 chitinous internal shell is a secondary development. In the 

 Gymnosomata, the primary shell is also cast off, but is not 

 replaced, and three girdles of cilia are developed on the sur- 

 face of the body. 1 



The Silurian genera Tentaculites, TTieca, Pterotheca, 

 Conularia, Ecculiomphalus, are referred to the Pteropoda, 

 but they differ much from all existing forms. Unquestionable 

 Pteropoda are not know earlier than the tertiary formations. 



The Branchiogasteropoda. — In all the members of this 



1 Gegenbaur, I. c. ; Krohn, " Beitvage zur Entwickelungsgesclnchte der 

 Pteropoden und Heteropoden," 1860; and Fol, "Etudes" ("Archives de 

 Zool. Experimentale," 1875 and 1876). 



