MOLLUSCA. 293 



chance being swept into a female Unio. There is no external 

 sexual distinction between a male and female mollusc. The 

 genital glands are branched structures, consisting of a gland on 

 each side of the middle line of the body. The tubes of these 

 glands are lined by a layer of specially modified cells, epithelial 

 cells, which develop into either ova or spermatozoa as the case 

 may be. The tailed spermatozoa fertilise the ova in the mantle 

 cavity of the female. One female Unio may produce as many 

 as 3,000,000 ova in a season. The ova collect between the gill 

 lamellae, where they undergo development, and here appear the 

 young larval Unios, which are known as Glochidmni larvse. 

 They eventually leave the parent and become attached to fish 

 and other objects, upon which they finish their development, 

 when they relinquish their hold and sink to the bottom of the 

 water, where the Glochidium becomes the adult Unio. Ex- 

 cretory organs are present, as the organs of Bojanus, acting as 

 kidneys in function, and connected with the pericardium as 

 well as the exterior. 



MoUusca have a distinct nervous system, typically composed 

 of three pairs of ganglia connected by commissures, each pair 

 forming a kind of nerve-centre. Some Mollusca have eyes, as 

 seen in Peotens and Helix (fig. 169, E), whilst auditory organs 

 are also found (otoliths). 



The Mollusca are divided into four groups — namely : 



1. Lamellibranohiata. 



2. Gasteropoda. 



3. Cephalopoda. 



4. Pteropoda. 



These are divided into two sections — 



A. Acephala (headless molluscs) = Lamellibranchiata (Mussels, 



&c.), or bivalves. 



B. Encephala (headed molluscs), or univalves and multi- 

 valves. 



The Encephala are provided with a curious masticatory 

 apparatus called the " odontophore '' : one part is a ribbon- 



