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 

 oro-an 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 Gyinnosomata the ganglia are more scattered, but 

 the arrangement of their nervous system needs reexamina- 

 tion. 



All the Pleropoda are provided vvith an ovotestis. This 

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

 and spermatozoa are developed. The spermatozoa make 

 their app3arance 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 recej^taculum 

 seminis and connected with a penis. 



The young of the Pteropoda leave the eg^ 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 Cymhidla, 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.* 



The Silurian genera Tentaculltes^ TJieca, Pterotheca^ 

 Conularia, EecuUomphalus, are referred to the Pteropoda, 

 but they differ much from all existing forms. Unquestionable 

 Pteropodx are not know earlier than the tertiary formations. 



The Bran-chiogasteropoda. — In all the members of this 



' Gesrenbaur, I. c. ; Krohn, " Beitrfige zur Entwickelungscfescliichte der 

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

 Zool. Exp^rimentale," 1875 and 1876). 



