iv LATEEAL MESODEEM 217 



Lateral Mesoderm. — The lateral mesoderm forms the lining of 

 the splanchnocoele. Its superficial layer persists throughout life 

 as the coelomic or peritoneal epithelium, while its deep surface 

 produces by proliferation abundant mesenchyme cells which forma con- 

 nective-tissue backing to the epithelium. The development of muscle- 

 fibres, which is so characteristic a feature of the coelomic lining in 

 the dorsal or myotomic region, is here to a great extent suppressed, 

 this portion of the mesoderm no longer playing any part in the 

 muscularization of the body-wall. It still however takes place in 

 restricted areas, smooth or striped muscle-fibres being developed in 

 those portions of the mesoderm which invest particular organs 

 such as heart and blood-vessels, alimentary canal with its appendages, 

 oviduct. The development of the musculature of the heart will 

 more suitably be treated in the chapter dealing with the vascular 

 system. As regards the muscles of the gut-wall we have little detailed 

 knowledge, what there is being related mainly to the musculature of 

 the skeletal elements contained in the visceral arches. 



In the larva of Lepidosiren the important point has been estab- 

 lished by Agar (190*7) that the sheath of muscle which forms the con- 

 strictor of the pharynx is of double origin, its ventral and larger 

 portion being a development of the splanchnic mesoderm covering 

 the pharynx, while its dorsal portion arises as an outgrowth from 

 one (y) or more of the occipital myotomes. The fact that muscular 

 tissue derived from myotomes may join the splanchnic muscle to 

 form part of the muscular sheath of the alimentary canal is of 

 importance (1) by impressing upon us that an apparently homo- 

 geneous muscular apparatus may really be heterogeneous — muscular- 

 ized from two quite distinct sources, and (2) by indicating the 

 possibility of splanchnic musculature being replaced by myotomic 

 or conversely. Obviously the muscular sheath just mentioned 

 might, by reduction of one or other of its component parts, become 

 purely myotomic or purely splanchnic. As will be gathered later 

 (Chap. VII.) the point is an important one from its bearing upon the 

 discussion of certain problems of morphology. 



Eenal Organs. — In triploblastic Metazoa the function of 

 excreting nitrogenous waste products is commonly carried out by 

 tubular organs to which Lankester (1877) gave the name nephridia. 

 Under this term were included the excretory tubes of Chaetopods, 

 Molluscs, Eotifers, Trematodes, Turbellarians and Vertebrates. 

 Subsequent research soon brought to light an important structural 

 difference as regards the inner ends of these nephridial tubes in 

 different groups of animals. In certain groups the tube possesses 

 at its inner end an open funnel or nephrostome 1 which leads 

 from the coelome into the cavity of the tubule, while in other 

 groups the inner end of the tubule is without any coelomic funnel 

 but is on the other hand provided with an arrangement of flame- 



1 Goodrich terms such funnels " coelomostomes " and uses the word nephrostome 

 in a special restricted sense. 



