TYPES OF SEXUAL DIFFERENCE 



133 



only in Chiton^ and possibly in Limax^ 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 the 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 DoUum has been described as Macgillivrayia^ 

 that of a Purpura as Chelotropis and Sinusigera^ that of A]jor- 

 rhais pes pelecani as Chiropteron^ that of Marsenia conspicua as 

 Brownia, Echinospira^ and Calcarella. 



Cephalopoda. — The embryonic development of the Cephalo- 

 poda is entirely distinct from that of all other MoUusca. 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 Qgg 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- 

 maphrodite type). 



In some cases — e.g. cer- 

 tain Pelecypoda — 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 Cephalopoda. 



G-asteropoda Amphineura (except Neomeniidae') . 

 G-asteropoda Prosohranchiata (except Valvata and some 



species of Marsenia). 

 Scaphopoda. 

 Many Pelecypoda. 



Fig. 48. — Two stages in the development 

 of Loligo vulgaris Lam.: «i, a^, first, 

 and 02, a2, second pairs of arms; hr, 

 branchiae, seen through m, mantle; e, 

 e, eyes ; fi, fins; fu, funnel ; v.s, vitelline 

 sac. (After Kowalewsky.) 



