IV LUMBRICUS 133 



The greater part of the thickness of the body-wall is occupied by 

 muscle fibres, which are arranged in two sharply defined layers, an outer 

 layer of circular {cm) and an inner layer of longitudinal fibres (l.m). 

 The latter have a very characteristic appearance in a transverse section, 

 owing to the contractile substance of each fibre being arranged in a curious 

 pinnate pattern. When the longitudinal muscles contract they cause 

 the body of the worm to shorten and thicken, while on the other hand 

 the circular muscles by their contraction cause the body to diminish in 

 diameter and consequently to increase in length. Amongst and around 

 the muscle fibres is a small amount of packing or connective tissue, 

 and finally the inner face of the body-wall is lined with a layer of extremely 

 thin flat cells — the coelomic epithelium (Fig. 64, c.e). 



The body-wall, the structure of which has been described, forms an 

 outer tube and within this there runs from end to end of the worm an 

 inner tube which is the enteron or ahmentary canal (Fig. 63, int). The 

 wall of this is composed of elements similar to those which constitute 

 the body-wall only arranged in reverse order. Internally is a layer of 

 columnar epithelium — the endoderm (end), outside this is a layer of 

 circular and then a layer of longitudinal muscle fibres — both very thin, 

 and finally a layer of coelomic epithelium (y.c). The tube so constituted 

 is seen on slitting open the body-wall of the worm to consist of several 

 regions of distinctive appearance (Fig. 65, A). 



The buccal cavity — the cavity of the mouth {b.c) — leads into the 

 somewhat ellipsoidal pharynx (ph) of characteristically furry appearance 

 owing to the presence of numerous slender muscles which radiate out 

 from it to the body-wall. These muscles when they contract serve to 

 dilate the pharynx and in this way produce a sucking action by which 

 food particles are drawn in through the mouth. The pharynx is con- 

 tinued back by a slender tube— the oesophagus or gullet (pes) — which about 

 segment XIV dilates to form the somewhat conical crop (c). This in turn 

 opens into the gizzard (g), about the same size as the crop but differing 

 from it in its walls being thick and hard, due to the exaggerated thickness 

 of the muscular layers. The gizzard stretches through about three 

 somites and then is continued onwards as the intestine (int) which extends 

 without further change to the anus at the hind end of the body. The 

 intestine is of characteristic appearance, its wall is thin and sacculated, 

 it has a brownish- yellow colour, owing to the peculiar nature of the 

 coelomic epithelium covering it, and along the mid-dorsal line its wall 

 projects inwards as a prominent fold — the typhlosole (Fig. 63, ty). 



The main functions of the enteron are concerned with the ingestion, 

 digestion, and assimilation, of the food. The food drawn into the 



