MOLLUSCA: GASTROPODA THECOSOMATA 

 AND GYMNOSOMATA 



By (the late) Anne L. Massy 

 (Plate XXXIX; text-fig. i) 



INTRODUCTION 



THE pelagic Mollusca, usually known as Pteropods, which form the subject of this 

 report, attracted at an early date the attention of ship captains and naturalist voyagers, 

 for they exist in the open sea far from any shelter other than that afforded by drifting 

 weed and sometimes occur in vast swarms. Individually they are very small. Indeed, 

 one of the larger species fully extended measures scarcely more than an inch. 



The Thecosomata are protected by fragile shells, which may be coiled, triangular or 

 needle-shaped. One family, the Cymbuliidae, possesses slipper-shaped gelatinous 

 conchae of great beauty. The Gymnosomata, as their name implies, are without shells. 

 All are hermaphrodite and some species of Gymnosomata have been observed to be very 

 precocious in reaching sexual maturity: thus, Pelseneer (1887, p. 49) alludes to a speci- 

 men of Clione flavescens (Ggbr.) that was able to lay eggs although it was hardly 2-0 mm. 

 in length and still possessed ciliated rings. Kwietniewski (1902, p. 17) states that in a 

 number of other species the sexual elements develop early. In 1917 I noted that speci- 

 mens of Pneiimodermopsis paucidens (Boas) from the west of Ireland, measuring only 

 1-2-5 '""f"- i^ length, had external accessory glands, and an individual measuring only 

 0-75 mm. had the penis evaginated (Massy, 1917, pp. 231-232). 



Diurnal migration is a well-established phenomenon in this group. 



Pteropods serve as food for some of our most valuable food fishes^ and for sea birds. 

 They are said also to form the food of whales, and as early as 1770 Cranz, in his Historie 

 von Gronland, named Clione " Walfischfrass " (food devoured greedily by the whale). 

 The earliest mention of the group appears to have been made in 1780 when Fabricius 

 produced his Fauna of Greenland and described Limacina helicina, Phipps, under the 

 name Argonaiita arctica. 



In the course of the Discovery Investigations many thousands of Pteropods have been 

 taken. A special study of the plankton has been made in southern waters near South 

 Georgia, the South Sandwich and the South Shetland Islands and the distribution of the 

 five species of Pteropods, to one or other of which the many individuals belonged, is 

 dealt with in a separate report. In warmer regions far fewer individuals were captured, 

 but thirty-one species belonging to eighteen genera were represented. 



1 Statistics in the Irish Fisheries Office of the stomachs of herring and mackerel taken on the. west coast 

 of Ireland show that Clione is a common food in spring and autumn. 



!\:i;^ 24 1932 



