PHYLUM MOLLUSCA 



685 



ccelomic epithelium. In the remainder of the body the space 

 between the ectoderm and the viscera is filled by the muscles 

 and connective-tissue. 



Digestive organs. — The mouth (Fig. 565, mth) .lies in the 

 middle line, just below the anterior adductor. On each side of ' 

 it are two triangular flaps, the internal and external labial palps 

 {I. int. pip., I. ext. pip.) ; the external palps unite with one another 

 in front of the mouth, forming an upper lip ; the internal palps are , 

 similarly united behind the mouth, forming a lower lip ; both are 

 ciliated externally. The mouth leads by a short gullet (Fig. 566, 

 gul.) into a large stomach (st.), which receives the ducts (d.d.^ of a 

 pair of irregular, dark-brown digestive glands (d.gl.). The intestine 



I. m- rch 



f^y*'^ dh.a. 



a.ouL 



'in. sbk 





-..C.int.g-l ' • ' - '■ -' ' 



t.i.nt.^lfo'-'-nt.gl^ /_ /^.ej^i^i f^d. 



Fig. 56.5. — Anodonta cygnea. The animal with most of the left mantle-lobe removed. 

 a. anus ; a. ad. anterior adductor ; a. r. anterior retractor ; au. auricle ; d. p. a. dorsal 

 pallial aperture ; ex. sph. exhalant siphon ; ft. foot ; in. sph. inhalant siphon ; kd. kidney ; 

 I. ext. gt. left external gill-lamina ; l. ext. pip. left external labial palp ; /. int. gl. left internal 

 gill-lamina ; I. int. pip. left internal labial palp ; I. m. cut edge of left niantle-lobe ; mth. 

 mouth ; p. ad. posterior adductor ; pc. pericardium ; p. r. posterior retractor ; jii'a. protractor ; 

 ret. rectum ; '/■, m. right mantle-lobe ; v. ventricle ; v. ra. visceral mass. 



(int.) is given off from the posterior end of the stomach, descends ' 

 into the visceral mass, where it is coiled upon itself, then ascends 

 parallel to its first portion, turns sharply backwards, and proceeds, 

 as the rectum {ret.), through the pericardium— where it traverses the 

 ventricle of the heart — and above the posterior adductor, finally 

 discharging by the anus {a.) into the exhalant siphon, or xloaca. 

 The wall of the rectum is produced into a longitudinal ridge, or 

 typhlosole {ty.), like that of the Earthworm, and two similar ridges 

 begin in the stomach and are continued into the first portion of 

 the intestine. The stomach contains, under certain conditions, 

 a gelatinous rod, the crystalline style. 



On each side is a single gill or ctenidium composed of two 

 plates or lamince, an inner and an outer. We have thus right outer 



