XII 



PHYLUM MOLLUSCA 



667 



between the two mantle-lobes, containing the gills, visceral mass, 

 and foot, is called the mantle-cavity. 



A single layer of epithelial cells covers the whole external sur- 

 face, i.e. the body proper, both surfaces of the mantle, the gills, 

 and foot ; that of the gills and of the inner surface of the mantle 

 (Fig. 574, ep. ?) is ciliated. Beneath the epidermis come connective 

 and muscular tissue, which occupy nearly the whole of the interior 

 of the body not taken up by the viscera, the ccelome being, as we 

 shall see, much reduced. The muscles are all unstriped, and are 

 arranged in distinct bands or sheets, many of them very large and 

 conspicuous. The largest are the anterior and posterior adductors' 

 (Figs. 573 and 575, a. ad., p. ad.), great cylindrical muscles, pass- 

 ing transversely across the body and inserted at either end on to 



I. m. 



FIG. 575. Anodonta cygnea. i'he 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 

 pallia! aperture ; ex. sph. exhalaut siphon'; .ft. foot ; in. sph. inhalant siphon ; kd. kidney ; 

 /. ext. al. left external gill-lamina ; 1. ext. pip', left external labial palp : /. int. gl. left internal 

 gill-lamina ; /. int. pip. left internal labial palp ; 1. m. cut edge of left mantle-lobe ; ruth. 

 mouth ; p. ad. posterior adductor ; pc. pericardium ; p. r. posterior retractor ; pro. pro- 

 tractor ; ret. rectum ; r. m. right mantle-lobe ; v. ventricle ; v. m. visceral mass. 



the valves of the shell, which are approximated by their con- 

 traction. Two muscles of much smaller size pass from the foot 

 to the shell, which they serve to draw back ; they are the anterior 

 (a. r.) and posterior (p. r.) retractors. A third muscle (prc.), 

 inserted on to the shell, close to the anterior adductor, has 

 its fibres spread fan-wise over the visceral mass, which it serves to 

 compress, thus forcing out the foot and acting as a protractor of 

 that organ. The substance of the foot itself consists of a complex 

 mass of fibres, the intrinsic muscles of the foot, many of which also 

 act as protractors. Lastly, all along the border of the mantle is a 

 ro\v of delicate pallial muscles (Fig. 573, pi. m.), which, by their 

 insertion into the shell, give rise to the pallial line already seen. 



The ccelome is reduced to a single ovoidal chamber, the peri- 

 cardium (Fig. 575, pc.), lying in the dorsal region of the body and 



