i 9 6 DISCOVERY REPORTS 



retroversa, Hsiao at no time found any pure females. The final stage in succession has evidently been 

 pushed back to occur simultaneously with the later part of the male phase, and the largest animals are 

 ' hermaphrodite females '. In the autumn bulimoides there were as many as 15 % °f pure females with 

 sperm absent, or present in the gonad only in the smallest traces. In these females, the penis is always 

 lost. Such a condition may perhaps occur also in the ' hermaphrodite females ' of retroversa, though 

 this appears to be nowhere explicitly stated by Hsiao. 



Within the family Limacinidae, it might be reasonable to expect that evolution— as in the great 

 group of opisthobranchs as a whole — has moved away from complete protandry, towards a telescoping 

 of the sexual succession in the direction of simultaneous hermaphroditism. 



One remaining species of Limacina, which dwells in cold waters, the very large, abyssal-planktonic 

 helicoides, which may reach a diameter of 12-5 mm., appears from Tesch's account (1946) to have 

 developed reproductive specializations not elsewhere found in the family. This species, as first noticed 

 by Bonnevie (1913), is viviparous and, at the female stage, developing eggs and small embryonic 

 replicas of the adult are found in the distended coils of the mucous gland. 



We may attempt to sum up the sexual development and succession in L. bulimoides in a diagram 

 (Fig. 16) expressing the stages in the maturation of the gonad and the condition of the accessory 

 genital apparatus against the growth of the shell. The lower panel represents the number of gonadial 

 whorls in relation to the total number of whorls of the animal. This is a feature easy to deter- 

 mine by external inspection. The gonad shows a regular increase up to 3^ whorls in the mature 

 female at stage 6. It should be noted that because of the increase in size from the apical to the body 

 whorl this relationship does not directly express the proportion of gonadial to other tissues. With 

 practice in the examination of stained sections, the viewing of the gonads of cleared whole animals 

 lightly stained in Ehrlich's haematoxylin yields accurate information as to the stage of sexual develop- 

 ment. Parallel with gonad growth, Fig. 16 shows the stages in gametogenesis corresponding to the 

 sexual phases established for this species in Table 7. The development of the male accessory organs is 

 expressed in terms of the maturity of the prostate gland as determined by the height of its epithelial 

 cells. In the female system, development is regarded as a function of the cell height of the epithelium 

 of the mucous gland, while the total diameter of the tubular receptaculum is shown on the same scale — 

 this organ increases in size not primarily by growth but by rapid inflation when the sperm is deposited 



at copulation. 



(iv) Discussion — Sex in Gastropoda 



With the description of sexual succession in a primitive opisthobranch and in a primitive pulmonate 

 and with a knowledge of the changes undergone by the gonad in some primitive prosobranchs, we 

 may venture very tentatively to reconstruct the evolution of sexuality in gastropods (see diagram, 

 Fig. 17). Let us suppose protandrous hermaphroditism to be the earliest condition in the gastropoda 

 at the point where, from evidence given elsewhere (Morton, 19536) the three great subclasses of 

 gastropods draw closest together. We may suppose this to be somewhere near the archaeogastropod 

 level of the prosobranchs. From such a condition, perhaps like that of Bacci's Fissurella nubecula 

 today, and without any development of genital ducts, we first find among living and archaic archaeo- 

 gastropods a tendency to the separation of the sexes in different individuals, which has probably been 

 best realized in living trochids. Then, with the emancipation of the renal genital duct from its excre- 

 tory function and the addition of a glandular genital duct developed from the pallial wall, the further 

 advances are possible. In almost all the rest of living prosobranchs there is a tendency to complete 

 separation of the sexes and bisexuality obtains in the great majority of species. Such cases of herma- 

 phrodite prosobranchs as do remain (such as the Calyptraeidae and the Valvatidae), assumed in the 

 past to be specialized, may on this interpretation be regarded as survivals of a primitive hermaphro- 



