ii2 ELEMENTARY BIOLOGY. [CHAP. 



The blood of the Anoddnta is colourless, and contains 

 colourless corpuscles, which resemble those of Man in struc- 

 ture and present the same Amcebiform movements. 



The nervous system of the Anadonta consists of three 

 pairs of yellow ganglia ; the cephalic, situated at the sides of 

 the mouth ; the pedal, placed in the foot ; and the parieto- 

 splanchnic, on the under face of the posterior adductor mus- 

 cle. They are united by commissural cords which connect 

 the cephalic ganglia with one another, and with the pedal 

 and parieto-splanchnic ganglia, respectively. The only sense 

 organs which have been discovered, are a pair of auditory 

 vesicles, connected by nervous cords with the pedal ganglia. 



The sexes are distinct. The testes and ovaria are similar 

 in character, being racemose glands, which, in the breeding 

 season, occupy a great part of the interior of the body. 

 There is one gland on each side, opening by a minute aper- 

 ture close to that of the organ of Bojanus. 



The spermatozoa have minute, short, rod-like bodies, to 

 which a long, filamentous, active cilium is attached, and, 

 thrown off in enormous numbers, make their way out with 

 the exhalent currents. 



The ova are spherical, and the vitelline membrane is pro- 

 duced at one point into a short open spout-like tube, with a 

 terminal aperture, the micropyle, through which, in all pro- 

 bability, the spermatozoon makes its entrance. When fully 

 formed, multitudes of these ova pass out of the oviducal 

 aperture and become lodged in the chambers of the gills, 

 particularly the external gill, which is frequently completely 

 distended by them. Here they are hatched, and give rise to 

 embryos, which are so wholly unlike the parent Anodonta, 

 that they were formerly thought to be parasites, and received 

 the name of Glochidium. The embryo Anodonta is provided 

 with a bivalve shell. Each valve has the form of an equi- 



