668 



ZOOLOGY 



SECT. 



containing the heart and part of the intestine ; it is lined by coelomic 

 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. 575, 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 

 (1. 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. 576, 

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



pc 



et.f>> 



kd 



U'.t 



VIG. 576. Anodonta cygnea. Dissection from the left side. a. anus ; a. aa. anterior 

 adductor ; a. ao. anterior aorta ; a. v. ap. auriculo-ventricular aperture ; bl. urinary bladder ; 

 e. pi. gn. cerebro-pleural ganglion ; d.d. duct of digestive gland ; d. gl. digestive gland ; 

 d. p. a. dorsal pallia! aperture ; ex. sph. exhalant siphon : ft. foot ; g. ap. genital aperture ; 

 gon. gonad ; gul. gullet ; i. 1. j. inter-lamellar junction : in. sph. inhalant siphon ; int. intes- 

 tine ; kd. kidney ; m. mantle ; mth. mouth ; p. ao. posterior aorta ; p. ad. posterior adductor ; 

 pc. pericardium ; pd. gn. pedal ganglion ; r. ap. renal aperture ; r. an. right auricle ; 

 ret. rectum ; r. p a. reno-pericardial aperture; st. stomach ; ty. typhlosole ; v. ventricle ; 

 v. gn. visceral ganglion ; w. t. water tubes ; x. aperture between right and left bladders. 



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

 (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 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 intestine. The stomach contains, under certain conditions, 

 a gelatinous rod, the crystalline style. 



