﻿94 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 102 



from the same mother. In fresh specimens from Guam, numerous, 

 irregularly shaped calcium-carbonate grains about 0.2 mjn. in diameter 

 have been found. It is possible that developing young feed on these 

 concretions. Occasionally a young snail may fail to escape from the 

 pouch, and in these cases no other young or eggs are found in the 

 pouch. These giants often grow to one-fifth the size of the mother 

 and probably rupture through the dorsal wall or cause the death of 

 the parent. 



Guam specimens kept in small vials of water gave birth to one young 

 on an average of every 12 hours. Three mature individuals were dis- 

 sected (Agana Spring), with the brood pouch contents noted as in 

 table 2. 



Reproduction in related groups. — The family Thiaridae exhibits an 

 interesting series of modifications within certain genera with regard 

 to manner of reproduction. It is believed that the more primitive 

 types are derivatives of the marine family Cerithiidae, a very similar 

 group in shell, radula, and animal characters. A number of 

 Cerithiidae are estuarine in habitat, and like their truly marine repre- 

 sentatives possess a veliger larval phase. Seshaiya (1940) has made 

 the remarkable discovery of a veliger stage in the fresh-water thiarid 

 Stenomelania crenulata Deshayes, in India. He found that the breed- 

 ing season commences about November and extends until the following 

 April. During this period the brood pouch on the back of the animal 

 contains thousands of developing eggs and a few veligers just escaped 

 from the vitelline membrane, but no juvenile, shelled forms. The 

 veliger stage, of about 2 weeks' duration, is spent in active swimming 

 in the river water. The full-grown veliger has the typical form found 

 in the life history of many marine gastropods and has two veliger 

 lobes provided with long cilia, by means of which it propels itself. 

 Seshaiya also noted the curious phenomenon found in many marine 

 invertebrates, that of spawning on the day previous to the full moon 

 and for the two succeeding days. 



A more advanced type of brooding of young within a neck pouch is 

 found in Thiara and Brotia. In the former there are fewer and much 

 larger-shelled young retained for a number of weeks within the pouch. 



A still more curious brooding modification is found in some other 

 thiarids such as Semisulcospira, of Japan, and the African (Lake 

 Tanganyika) genera Tiphohia, Tanganyicia, and Nassopsis (Moore, 

 1898) . Brooding takes place in the enlarged oviduct, which is welded 

 to the mantle. In the case of Semisulcospira the shelled young are 

 rather small and all of the same size. Spermatozoa have been ob- 

 served in male specimens. 



The habit of laying eggs rather than brooding is considered to be 

 a more nearly perfect adaptation to fresh-water conditions. 

 Paludonms^ of Ceylon, India, the Philippine Islands, and southern 



