THE ANATOMY. MUSCULAR LAYERS 17 



other family of Oligochaeta ; but it does not seem to be too much to assume that 

 they are really sense-organs, though their function must remain a matter of doubt. 



§ 7. Muscular Layers. In nearly all Oligochaeta the muscular layers of the 

 parietes are arranged in two layers. There is an outer circular and an inner longi- 

 tudinal. It is a little difficult in so delicate a form as Aeolosoma to recognize that 

 both these layers are present. The only exception to this rule seems to be the 

 Enchytraeid genus Fridericia, where there are two separate layers of longitudinally 

 running fibres (as well as the circular layer). 



In Lumbricus (see especially Cerfontaink), the individual fibres of the circular 

 coat form a layer of some thickness, variable, however, and particularly thin upon 

 the intersegmental regions. The fibres are imbedded in a granular nucleated substance, 

 and have a more or less strongly-marked columnar arrangement. The individual 

 fibres are long and pointed at both extremities — a statement which applies to the 

 muscular fibres of any part of the body — and longitudinally striate. This striation 

 is apparently due to the presence of moniliform fibrils which makes up the substance 

 of the fibre, and which sometimes give it a transversely striate appearance ; these 

 fibrils are imbedded in a clear interfibrillar fluid, which is often particularly plain in 

 the axis of the fibre, giving it a hollow appearance. 



The 'leech-like character' of the muscular fibres of the earthworm was first 

 pointed out by Ratzel, who, however, believed that only certain parts of the body- 

 wall had muscles of this kind, whereas Cekfontaine showed that they are general. 



The granular stroma in which the fibres are imbedded shows no cell outlines ; 

 it contains clear spaces which are probably the 'lymphatic' spaces referred to on p. 30. 

 The longitudinal muscles of the earthworm form a layer of much greater diameter 

 than the circular layer. In many species, as was first pointed out by CLAPAEiiDE 

 (1), a peculiar bipinnate arrangement of the fibres exists. The individual fibres 

 .appear to be attached on either side of a central rhachis formed by a septum 

 of connective tissue. This peculiar appearance does not exist in all species of 

 Lumbricidae, but it does occur in a few earthworms not belonging to that family. 

 Ude (3) explained the appearance not as due to a series of connective tissue 

 septa with fibres regularly given ofi" on both sides, but as due to a series of 

 compartments to the walls of which the fibres are attached all round. Cekfontaine 

 proved, by histological methods more refined than those open to Claparede, that 

 there is really no difference from the circular layer ; similar fibres are imbedded in 

 an identical granular stroma ; the regularity of their arrangement, however, producing 

 the appearance, wrongly interpreted by CLAPAEtDE, is more marked than in the 

 circular muscles, where, nevertheless, it exists. Here and there, and more particularly 



D 



