208 ESSENTIALS OF ZOOLOGY 



segmental muscular system is practically lost. The muscles 

 of the body and limbs become modified in such a way that 

 homologies with the muscles of the higher Craniates are at 

 once suggested, and, indeed, the nomenclature of human 

 anatomy is used in describing them. This will be plain if, as 

 soon as the frog is pinned out in the dissecting dish and the 

 skin reflected from the ventral surface, a glance be given to 

 the muscles of the body wall. The recti meet medianly in a 

 linea alba and are interrupted by transverse lines of connective 

 tissue. Lateral to them are the internal and the external 

 oblique muscles. In front the large pectoralis muscles are 

 prominent, and the myohyoid muscle which occupies the floor 

 of the mouth. The upper segments of the hinder limbs are 

 very muscular, and the muscles are defined by names familiar 

 to the medical student. 



The cavity of the splanchnocoel gradually extends ventral- 

 wards, and in doing so separates the mesoderm into parietal 

 and visceral layers. The body cavity thus formed is resolved 

 into the forward pericardium and the posterior pleuro- 

 peritoneal cavity. As soon as the body wall is cut through, 

 the muscular layers being noted and the abdominal vein 

 isolated, the pigmented peritoneum is seen as a membrane 

 which lines the body cavity and which is reflected dorsally 

 to support and cover the viscera. It is a closed cavity com- 

 pletely lining the internal surface of the body wall, and re- 

 flected in two layers dorsally to form the mesentery, which 

 diverge to receive the alimentary canal and other viscera. 

 Anteriorly it is reflected to form a covering for each of the 

 lungs, a visceral pleura thus being produced, and it comes 

 here into contact with the pericardium. In the female the 

 peritoneum is pierced anteriorly by the internal openings of 

 the oviduct. The pericardium is also a closed sac, so disposed 

 that the inner or visceral layer is closely attached to the heart, 

 and that anteriorly it is reflected to form the parietal layer, 

 which also completely envelops the heart. These membranes 

 provide smooth moist adposed surfaces externally to the 

 viscera, and thus the movements of the latter are facilitated. 



The myocoel develops a sclerotome which yields mesen- 

 chyme to ensheath the notochord and the nerve cord, and it is 



