184 DISCOVERY REPORTS 



single season there is a well-spaced successive hermaphroditism, male to female. Limacina represents 

 the first primitive opisthobranch so investigated, and its life-history may be tentatively regarded in all 

 respects as typical of the early gastropods. The specialization of atypical sperm-formation has not 

 been developed. Hsiao looked for the representatives of 'nurse sperms' in Limacina and not sur- 

 prisingly did not find them. The masses of residual cytoplasm observed by him are too indefinite to 

 suggest any such anomaly and have not been found in Limacina bulimoides at all. 



(i) The Gonad and Genital tract 



We have now a description of a primitive genital tract in a pulmonate (Morton, 19540) and in a 

 primitive opisthobranch, from the earlier work of Guiart (1901), while a forthcoming account by 

 Lloyd will make clear the condition in several bullomorphs. In Limacina we must look for features 

 characteristic of primitive tectibranchs, with resemblances on the one hand to early pulmonates and 

 on the other to prosobranchs. In primitive opisthobranchs we are probably near a point where the 

 three major groups of gastropods draw very close together. 



The gonad consists of a long, tubular sac occupying in the mature female 3^ visceral whorls and the 

 whole of the apex of the animal. It is not subdivided and bears in all its stages a general resemblance 

 to that of L. retroversa. It leads forward alongside the liver into a narrow hermaphrodite duct. This 

 opens into the glandular genital tract, which is the pallial genital duct of Fretter (1946), and lies on the 

 floor of the mantle cavity, at later stages bulging strongly into it. It consists of an albumen gland and 

 a mucous gland. From the albumen gland, in the mature female there runs back alongside the stomach 

 a sausage-shaped receptaculum seminis, lying dorsally on the first visceral whorl and attaining almost 

 the size of the stomach. The common genital aperture opens on the right side rather far forward in 

 the pallial cavity, and from it a ciliated fold leads forward to the secondary male aperture on the head 

 at the base of the right wing. The male aperture leads into a highly complicated invaginated penis in 

 front of and quite separate from the pallial tract; and with the penis is closely associated a stout, 

 tubular prostate gland. 



Condition of the gonad and sexual stages 



Hsiao (19390) divided his population into those with sexually undifferentiated gonads, those with 

 'pure male' gonads (designated 'functional males'), 'functional hermaphrodites', and finally 'herma- 

 phrodite females'. It was recognized that no clear line of demarcation separated the males and 

 females, but that they merged into a continuous series. No pure females were found among L. retro- 

 versa, and functional females were held to be those where more than 75 % of the gonadial contents 

 were oocytes or eggs. In bulimoides, however, it is possible to recognize a pure female phase. The 

 gonad in animals of more than 1 -9 mm. shell height is usually cleared of all but a few lingering sperms. 

 The female duct is now highly developed, the penis is lost and the sperm sac is full after copulation. 

 The sexual succession is thus better spaced and the later stages further separated in bulimoides than 

 in retroversa. 



We may list in Table 7 the sexual phases and their characteristics as recognized in L. bulimoides. 



For the three largest samples of L. bulimoides comprising 5800 individuals from stations WS 

 996 and 997, the following data were obtained (Table 8) for the percentage distribution of sexual 

 stages with size. Seven size groups separated by 0-2 mm. intervals were obtained by breaking up the 

 three samples by the use of graded and measured pipettes, and the results afterwards carefully checked 

 by the measurement of representative samples from each group. From each size group a sample of 

 approximately fifty individuals was then examined after bulk staining with Ehrlich's haematoxylin and 



