12 Guide to the Mollusca. 



Renal organs (kidneys) are found in all molluscs, usually in 

 intimate connexion with the pericardium and reproductive 

 organs. In certain forms they act as genital ducts as well as 

 kidneys. In general structure they are glandular sacs, the walls 

 of which are increased by extensive folding. The liver some- 

 times acts as an excretory organ, and certain of its cells may be 

 specialized for that purpose. 



The genital ducts may be simple or may have an elaborate 

 apparatus of accessory glands and other organs. In the Aplaco- 

 phora and certain Gastropoda there are no genital ducts and the 

 germ cells are liberated through the renal ducts. 



Reproduction and Development. 



The sexes are usually separate, but hermaphroditism occurs in 

 nearly all the classes, and the Euthyneura among the Gastropoda 

 are exclusively hermaphrodite. In a few molluscs the animal 

 passes through a regular sexual cycle, being male at one period 

 and female at another. This occurs in the Limpet (Patella), in 

 Crepidula and probably in Aplysia among Gastropoda, and in 

 the Common Oyster among Lamellibranchia. Hermaphrodite 

 species are usually cross-fertilized, but self-fertilization sometimes 

 occurs. Only one case of parthenogenesis is known, that of the 

 Gastropod Paludestrina je?ikinsi. The males and females of 

 bisexual forms are sometimes distinguished by the size and shape 

 of their shells. 



Fertilization is either external or internal. The former is 

 usually the rule among marine forms, in which the number of eggs 

 discharged from the ovary may be enormous, the Common Oyster 

 depositing as many as 60,000,000. In most aquatic Mollusca 

 the young pass through a series of free-swimming larval stages. 

 The first stage is usually that known as the Trochosphere, which 

 resembles very closely the earliest larval stage of certain Annelid 

 worms. Later a lobed swimming-organ, the velum, is developed 

 and the larva is termed a Veliger. In all terrestrial molluscs and 

 in certain aquatic forms, such as some Cephalopoda, the eggs are 

 few and large and develop directly, the larval stages being 

 suppressed. 



The large eggs of some terrestrial molluscs may have a calcareous 

 shell. In numerous cases the eggs are laid in clusters or in 

 ribbons, which take on various shapes (Wall-cases B and E). 



There is a great variety of parental care among Mollusca, from 

 a mere temporary association with the parent, as in some 

 Gastropods in which the eggs are laid on the shell, to con- 

 tinuous incubation in special brood-pouches in the mother's body 

 as in the Pond-Snail Vivipara. In certain Lamellibranchia — 

 the Unionidae, Cyrenidae, &c— the young are incubated between 



