128 INVERTEBRATE MORPEOLOOY. 



majority of forms, a cavity, the enteron (cZ), the cells lining 

 the walls of this being differentiated into a digestive epithe- 

 lium or endoderm. The space between the enteron and the 

 basement-membrane is occupied by the mesoderm, consisting 

 peripherally of compact layers of circular and longitudinal 

 muscle-fibres (m), while below these it forms a mass of nu- 

 cleated cells, usually vacuolated so as to resemble a network 

 of fibres enclosing spaces and constituting the parenchyma (p). 

 It is traversed by dorso-ventral ' muscle-fibres and has im- 

 bedded in it various organs most of which are further dif- 

 ferentiations of this middle germ-layer. These two layers, 

 the endoderm and mesoderm, are together comparable with 

 the inner layer of the Coelenterates, the mes-endoderm, and 

 when the enteron exists it communicates with the exterior, as 

 in that group, by a single opening, the mouth, the Nemer- 

 teans only, the most highly organized class of the Platyhel- 

 minths, possessing a second opening, the anus. 



These homologies are, however, associated with a com- 

 plexity of organization unrepresented in the Ccelenterates. 

 The Platyhelminths all present a typical bilaterality of form, 

 and show furthermore a well-marked antero-posterior as well 

 as, in most cases, a dorso-ventral differentiation. The' body is 

 usually flattened and more or less vermiform, whence the name 

 of the group, and is adapted to a creeping habit, certain para- 

 sitic forms, and some Nemerteans which live buried in sand, 

 being the only forms not presenting such a mode of life. 



The greatest contrast to what occurs in the Coelenterates 

 however, is presented by the development of compact organs. 

 The nervous system is no longer an altogether diffuse tissue, 

 scattered in a thin layer throughout the body, but a large 

 number of ganglion-cells are aggregated into a compact mass, 

 the brain, embedded in the mesoderm parenchyma near the 

 anterior end of the body, and from this there pass backwards 

 two or more longitudinal cords of nerve-fibres which give off 

 branches extending to all parts of the body and forming a 

 network below the basement-membrane from which the pe- 

 ripheral muscles derive their nerve-supply. In some cases 

 nerves have been observed to pass from this network through 

 the basement-membrane to come into connection apparently 



