636 



ZOOLOGY 



SECT. 



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. 527, mth.) lies in the 

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

 it are two triangular flaps, the internal .(Lint, pip.) and external 

 (1. ext. pip.), labial palps; the external palps unite with one another 

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

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

 ciliated externally. The mouth leads by a short gullet (Fig. 528, 

 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 



ct.r 



I. rrv rcl 



a. ad. 



FIG. 527. Anodonta cygnea. The animal with most of the left mantle-lobe removed ; 

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

 pallial aperture; tx. *ph. exhalant siphon;/?, foot; in. sph. inhalant siphon; M. kidney; 

 1. ext. gl. left external gill ; 1. ext. pip. left external labial palp ; I. -int. <jl. left internal gill ; 

 1. int. pip. left internal labial palp ; 1. ui. cut edge of left mantle-lobe ; mth. mouth ; p. ad. pos- 

 terior adductor ; pc. pericardium ; p. r. posterior retractor ; prc. protractor ; ret. rectum ; /. ni. 

 right mantle-lobe ; r. ventricle ; v. m. 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, arid 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 amis (a.) into the exhalant siphon, or cloaca. 

 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 intestihe. The stomach contains, at certain seasons of the 

 year, a gelatinous rod, the crystalline style. 



The gills consist, as we have seen, of two plate-like bodies on 

 each side between the visceral mass and the mantle : we have thus 



