TYPES OF SEXUAL DIFFERENCE 



133 



Chiton, and possil>ly in Li max, that the primitive shell-sac is 

 retained and developed into the final shell-forming area, which 

 is much wider, and extends to the edges of the mantle. Within 

 the velar area first appear tiie rudiments of the tentacles and eyes ; 

 when these become developed the velum atrophies and disappears. 

 Several of these veligers when captured in the open sea have 

 been mistaken for perfect forms, and have been described as such. 

 Thus the larva of Dolmm has been described as Macgillivi-ayia, 

 that of a Purpura as Chelotropis and Siiiusigera, that of 

 Aporrhais pes pelccani us, Chiropteron, that oi 3farscnia conspicua 

 as Broivnia, Echinospira, and Calcarclla. 



Cephalopoda. — The endjryonic development of tlie Cephalopoda 

 is entirely distinct from that of all other Mollusca. The 

 segmentation of the vitellus 

 is partial, and the embryo is 

 furnished with a vitelline 

 sac, which is very large in 

 the majority of cases (Fig. 

 48). There is no free- 

 swimming stage, but the 

 embryo emerges from the 

 egg fully developed. 



Differences of Sex. — In 

 the Mollusca there are two 

 main types of sexual differ- 

 ence : (i) sexes separate (dioe- 

 cious type), (ii) sexes united 

 in the same individual (her- 

 Viaphrodite type). 



In some cases — e.g. certain I'elecypoda — what is practically 

 a third type occurs. The animal is hermaphrodite, but the male 

 and female elements are not developed simultaneously, i.e. the 

 same individual is at one time female, at another male. 

 1. The sexes are separate in 



All Cephcdopoda. 



Gasteropoda Amphineura (except Neomeiiiidae). 



Gasteropodct Prosohrancliiatct (except Vcdvata and some 

 species of Marsenia). 



tSrnpli opoda. 



I\Iany Pclccypoda. 



Fig. 48. — Two stages in the development of 

 Loligo vulgaris Lam. : aj, «i, first, and «o, «2, 

 second pairs of arms ; 6r, branchiae, seen 

 througli m, mantle ; e, e, eyes ; fi, tins ; fv, 

 funnel ; v.s, vitelline sac. (After Kowal- 

 ewsky. ) 



