312 ELEMENTARY BIOLOGY. [CHAP. 



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 

 muscle. They are united by commissural cords which con- 

 nect the cephalic ganglia with one another, and with the 

 pedal and parieto-splanchnic ganglia, respectively. Traces 

 of a small fourth pair of ganglia may be obvious on the 

 cerebro-splanchnic commissures, near the anterior end of 

 the pericardium. As the parieto-splanchnic ganglia are 

 immediately connected with a patch of sensiferous epi- 

 thelium in the roof of the inhalent siphon, they are sometimes 

 regarded as olfactory. The only other sense organs which 

 have been discovered, are a pair of auditory vesicles, con- 

 nected 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; they 

 are thrown off in enormous numbers, and make their way 

 out with the exhalent currents. 



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

 cient at one point, leaving a terminal aperture or micropyle, 

 through which, in all probability, the spermatozoon makes its 

 entrance. When fully formed,. multitudes of these ova pass 

 out of the oviducal apertures and become lodged in the 

 chambers of the gills, particularly the external ones, which 

 during autumn and winter are completely distended by 

 them and the embryos to which they give rise. 



Segmentation is holoblastic, and the early developmental 

 phases resemble, in their main features, those already de- 



