ECOLOGY— DEPTH DISTRIBUTION 183 



2 and 3. Two very juvenile larvae, not long out of the egg. The body is colourless, the eyes sessile 

 and the arms very short and subequal. Beyond the position of these larvae in the Architeuthacea, it is 

 possible to establish very little about them. 



Occurrence. WS 978, 350-500 m., one specimen, c. 3-5 mm.; WS 978, 250-500 m. (second haul), 

 one specimen, c. 2-5 mm. 



4. A larval histioteuthid, agreeing largely with the figure of a young stage of Calliteuthis reversa, 

 given by Naef (1923), p. 355, fig. 174: 'Naef was working in the Mediterranean on two histioteuthids 

 which he could separate because there are only two adult species known in that area. In the Atlantic 

 there are at least six species which may have larvae of this kind.' 



Occurrence. WS 1091, surface to 100 m., one specimen, c. 75 mm. 



II. THE REPRODUCTIVE SYSTEM AND SEXUAL SUCCESSION 



OF LIMACINA BULIMOIDES 



It is to the work of Hsiao (19390, b) that we owe almost our whole knowledge of reproduction in 

 Limacina. He gave an account of the reproductive system in L. retroversa and established that this 

 animal is a protandrous hermaphrodite passing after a sexually undifferentiated period through a male 

 stage, succeeded by a phase that is predominantly female. He worked with sufficiently large numbers 

 to establish the incidence of sex phases among the size groups in a population at different times of the 

 year. A knowledge of breeding in an ecological dominant like Limacina has two special advantages: 

 we are able to trace the elements in a population made up of mixed components not always with the 

 same relation between sex and size (see Redfield, 1939), and, with information about the mode of life 

 and conditions of reproduction, we are able to predict the probable seasonal movements and activity 

 of a given population. The Benguela material of L. bulimoides is especially good for the study of a large 

 population of a single origin with an apparently uniform sexual sequence. We still lack comparative 

 data for any other month, but— taken in the breeding month of March— the autumn Benguela 

 material contains a good proportion of mature females and a complete section of other age groups. 

 It is hoped in a second report to examine the late winter or early spring material from the second 



survey. 



Hsiao gave a rather brief account of the structure of the reproductive system with little histological 

 or structural detail. Unfortunately, from the literature then available he was not able to take a com- 

 parative view of Limacina in its relation to the other primitive opisthobranchs. The present material 

 of bulimoides, though preserved in routine formalin, was found to be good enough in many cases for 

 a study of histology and enabled a fuller report of the genital system to be prepared. The lack of living 

 material was unfortunate, yet with such a large number of stages and with living British retroversa for 

 comparison it was felt justifiable to redescribe the whole system. 



Protandrous hermaphroditism may well have been the original condition in the Gastropoda. In 

 prosobranchs at the most primitive level it has been found by Orton (1909) in Patella and by Bacci 

 (1947 a, b) in Fissurella nubecula and in Patella coerulea. In mesogastropods, hermaphroditism survives 

 only in rather isolated cases, such as Valvata and Crepidnla. We may regard the Mesogastropoda, 

 especially in their frequent atypical spermatogenesis, as sexually specialized gastropods. In the 

 pulmonates and opisthobranchs previously known, hermaphroditism is of the simultaneous type and 

 it appears, as for example in Helix and in higher nudibranchs, that the stages have been telescoped 

 together. Morton (19546) presents evidence that the opisthobranchs and pulmonates came off close 

 together from a prosobranch stock below the level of the present day Mesogastropoda, and in a study 

 of the ellobiids he gives the first account of reproductive succession in a primitive pulmonate. In a 



