Xiv INTRODUCTION. 



chiefly owing to the lack of properly preserved material for 

 investigation ; it may not, however, be out of place to here insert 

 a few biuomic notes which concern the families generally, these 

 being grouped under the various headings as below : - 



1 . Breeding Habits. 



2. Larval Stages. 



3: Development from Post- Larval Stages. 



4. Habitat and Mode of Life. 



5. Movements, Locomotion, and Dispersal. 



6. Economic Uses. 



1. Breeding Habits. Generally speaking the sexes in both the 

 freshwater Gastropoda and Pelecypoda are distinct, though here 

 and there examples of herinaphroditism are known to exist, as, 

 for instance, in the case of certain species o Anodonta. 



In the former case, the sexes can frequently be distinguished 

 by the form of the shell, that of the female being of a larger size 

 and generally more inflated than is that of the male. In the 

 Gastropoda this is especially to be remarked in the Viviparidae, 

 while in the Pelecypoda it is chiefly evident in the LTmonidae, the 

 female in this group being noticeably broader than the male. 



In the fluviatile Gastropod families sexual contact usually takes 

 place, ova as a result being deposited in the majority of instances., 

 though in the case of the Tiarida? and Viviparidse, as the name of 

 the latter would imply, the young are produced alive. 



In the Pelecypoda, however, union of the sexes does not take 

 place, the male at certain seasons in the year freely discharging 

 the spermatozoa into the water, these being introduced into the 

 inhalent siphons of the female by means of currents set up by 

 ciliary movements, fertilization taking place either in the oviduct 

 itself or else in specialized spaces of the mantle cavity. 



The quantity of eggs or young produced by the different 

 families varies enormously, this, while attaining in the Pelecypoda 

 to thousands, and sometimes even to hundreds of thousands, as in 

 the case of certain species of Unio and Anodonta, falls in Planorlis 

 and Limncea to anything from twenty to a hundred, and in Ancylus 

 to such a small total as five or six only, while Vivipara and Tiara 

 average not more than about fifteen individuals at a time. 



In Tiara the embryos are developed in a marsupium which is 

 formed by an infolding of the skin near the base of the right 

 tentacle, while in the Unionidae, CyrenidaB, and some other 

 Pelecypoda development takes place in the spaces between the 

 folds of the gills where, in the Cyrenida?, special marsupia exist 

 for their reception. 



2. Larval stages. In the fluviatile Gastropoda at birtli the 

 animal is generally more or less similar to that of the half-grown 

 or adult state, though the shell differs considerably ; in the earlier 

 stages it is exogastric or coiled forward over the head of the 

 animal, but rapidly assumes the normal spiral of the adult. In 



