338 ELEMENTARY BIOLOGY. [CHAP. 



chial one, except through the gill-slits. Its dorsal 

 prolongation (cf. Sect. B. d), seen, in section, as 

 a small cavity overlying the rectum. 

 8. The parieto-splanchnic ganglia; two yellow bo- 

 dies confluent in the midddle line, buried up in 

 a connective tissue sheath, immediately beneath 

 the posterior adductor muscle. 



d. Obliquely backwards, through the stomach and 



first coil of the intestine. Examine from behind. 

 a. The stomach; a spacidus sac, passing into the 

 intestine below. The orifice of the gullet is 

 seen as a wide transverse aperture, situated near 

 its roof. Immediately below this there is a cor- 

 responding depression, which receives the main 

 ducts of the digestive gland — two enormous tubes 

 admitting the seeker with ease; note the pre- 

 sence of a series of lesser ducts. 



fi. The intestine; passing straight down in the middle 

 line. Note the typhlosole; a median ingrowth of 

 the lining membrane of its front wall. 



y. The labial palps. These, like the gills, are con- 

 fluent — externally with the pallial-lobes, inter- 

 nally with each other and the body wall; their 

 free edges enclose a ventral ciliated groove. 



e. Through the anterior adductor muscle, immediately 



in front of the mouth. 



u. The adductor muscle. Cf. c. a. 



/3. The mouth; median, transversely elongated. It 

 lies immediately beneath the adductor muscle, 

 and is embraced on all sides by the labial palps, 

 which are confluent round it. Note that they 



