436 THE ANATOMY OF INVERTEBRATED ANIMALS. 



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

 branchiate, sometimes prosohranchiate. The branches of the 

 aortic trunk soon terminate in lacuna?, by which the blood is 

 conveyed back to the w^alls 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 

 paricardial sinus. 



The Thecosomata have the principal ganglia concentrated 

 around the gullet — the cerebral ganglia being lateral, and 

 united by a long commissure. 



In the Gyninosomata the ganglia are more scattered, but 

 the arrangement of their nervous system needs reexamina- 

 tion. 



All the Pteropodci are provided with an ovotestis. This 

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

 and spermatozoa are developed. The spermatozoa make 

 their appaarance at the closed end of the cas3um and accumu- 

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

 tissue of the cascum, somewhat lower down; nevertheless 

 fecundation does no? 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 tarmination of which may be provided with a recejytaculuin 

 seminis and connected with a penis. 



The young of the Pteropoda leave the Qgg provided with 

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

 operculum. In most of the Thscosom^Ua the shell is re- 

 tained and forms the commencement of that of the adult, 

 while the vela disappear and the epipodia are developed. 

 In Gymhulla, the primary external shell is shed and the 

 chitinous internal shell is a secondary development. In the 

 Gymnosoni'ita^ 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 Tentacrdltes, Tlieca^ Pterotheca^ 

 Conidaria^ Eccidlomphalus^ are referred to the Pteropoda^ 

 but they differ much from all existing forms. Unquestionable 

 Pteropodx are not know earlier than the tertiary formations. 



The Braxchiogasteropoda. — In all the members of this 



' Gejrenbaur, I. c. ; Krohn, " Beitrasrc zur Entwickelungssreschichte der 

 Pteropoden uiid Ileteropoden," 18<")0 ; and Fol, "Etudes" ("Archives do 

 Zool. Experimentale," 1875 and 1876). 



