MUSCLES OF VEEjSIES. 



143 



surfaces therefore directed inwards and outwards. In either case 

 the muscles are grouped in particular ways. They are separated 

 into two lateral masses by a dorsal and median line, formed by 

 other tissues ; and these masses consist of fibres lying directly on 

 one another (Grordius, Trichocephalus) . In the majority of 

 Nemathelminthes there is a further differentiation, due to the inter- 

 position of other organs at eacli side of the dermo-muscular tube. 

 This lateral line (Fig. 61, A r) is, in very many Nematodes, 



Fig. 61. Transverse section of Ascaris Inmbricoides, A, and of Hirudo, B 



c Cuticular layer, m Muscular layer, r Lateral line with the excretory organ. 



J? P Upper and lower median line, p ' Oblique fibres, v Enteron. d Dorsal. I Lateral 



vascular trunk, s Vesicle of the excretory organ, n Ventral nerve-chord. 



enlarged into a lateral tract, which is more or less developed; it 

 is present in the Chsetognathi also. 



3) The muscular system of the body consists of a layer of 

 external circular, and internal longitudinal fibres. Neither are 

 separated into distinct tracts in the Gephyrea, and Acanthocephali, 

 although in the former the separate longitudinal or transverse 

 muscular bands are frequently placed at some distance from one 

 another. On the other hand, the Annelides, owing to the arrange- 

 ment of the longitudinal fibres into two dorsal and two ventral 

 layers, have a distinct lateral field or groove ; the longitudinal layer 

 is the thicker. A layer of transverse fibres, generally represented 

 by distinct bundles, passes from the ventral median line to the 

 lateral grooves. 



In addition to these muscles, which are present throughout the 

 body, there are also separate muscles for special organs. We need 

 only mention here the muscles which move the bundles of setse, and 

 which are probably nothing more than fibres separated from the 

 muscular mass, which extends over the whole body. 



The suckers found in the Trematoda, Cestoda, and Hirudinea, 

 are special differentiations of the dermo-muscular tube, which agree 

 with one another in all the essential points of their structure. 



