32 COMPARATIVE ANATOMY 



Internal to the circular layer is a much thicker layer of 

 longitudinal muscles, imbedded, like those of the circular 

 layer, in a granular substance. The muscle fibres are identical 

 in structure with those of the circular layer and are arranged 

 in such regular rows that the layer appears to consist of a 

 series of radial connective tissue septa with the muscle fibres 

 attached on either side of every septum, looking in transverse 

 section like the barbs of a feather. The layer of longitudinal 

 muscles is much thicker than the circular layer. Internal to 

 the longitudinal muscles is a layer of pavement epithelium 

 forming the lining of the coelomic cavity. 



The gut lies free in the coelomic space and is not attached 

 to the body- wall by vertical mesenteries, though a suspensory 

 fold connects it in the mid-ventral line with the sub-intestinal 

 blood-vessel. (The section drawn in fig. 7 passes close to a 

 septum, and the strands of tissue passing from the body-wall 

 to the gut on the dorsal side are not mesenteries but folds of 

 the septum.) The coelomic epithelium covering the intestine 

 is modified to form the peculiar chloragogen cells whose 

 characters have already been noted. Similar chloragogen 

 cells surround the supra-intestinal blood-vessel, and are con- 

 tinued downwards into the fold of the typhlosole. Beneath 

 the chloragogen cells, which are, it must be remembered, 

 coelomic epithelium, is the muscular coat of the intestine, 

 chiefly composed of circular muscle fibres, the scanty longi- 

 tudinal fibres lying internal to the circular layer. The cavity 

 of the intestine is lined by an epithelial layer consisting of a 

 single layer of ciliated columnar cells. The structure of the 

 ventral nerve cord has already been described. 



