132 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



C.JTl. 



body, except on its ventral side, towards its anterior end. 'This thicken- 

 ing is the clltellum ; it varies somewhat in position in different species 

 of earthworm (Somites XXXII-XXXVII in L. herculeus, one of our 

 commonest large earthworms) and as will be seen later it performs an 

 important function in connexion with reproduction. 



The body-wall of the worm is, as may be seen by examining trans- 

 verse sections with the microscope, of complicated structure (Fig. 63). 

 Externally is the ectoderm or epidermis (Figs. 63 and 64, ec) a layer of 

 epithelium, the individual cells of which are columnar in shape and have 

 the superficial layer of their cytoplasm condensed to form the cuticle 

 which passes uninterruptedly from cell to cell. Here and there may be 



seen a gland-cell — its 

 cytoplasm laden with 

 drops of secretion, its 

 nucleus situated deep 

 down towards its inner 

 end, and its outer end 

 tapering off to a com- 

 paratively narrow tip 

 which is devoid of 

 cuticle. The cuticle 

 is thus incomplete, 

 or perforated by a 

 minute pore over each 

 gland-cell, so that the 

 secretion passes away 

 readily on to the outer 

 surface of the skin 

 which it serves to 

 keep moist. Should the section pass longitudinally through a chaeta 

 (Fig. 64) it may demonstrate the interesting fact that the chaeta 

 is simply a small patch of cuticle enormously thickened so that it 

 projects on the one hand outwards beyond the general surface and, 

 on the other, downwards into the thickness of the body-wall. The 

 inwardly projecting part of the chaeta is ensheathed in an epidermal 

 pocket — the cliaeta sac — and the chaeta is in fact the cuticle secreted 

 by the epidermis forming the chaeta sac. Chaetae are liable to be worn 

 out and shed, and in order to provide for this contingency a young reserve 

 chaeta is formed by an outgrowth of the main chaeta sac (Fig. 64). If 

 the main chaeta is lost, the reserve chaeta commences to grow actively 

 and soon takes its place. 



Fig. 64. 



Portion of transverse section of an earthworm passing through 

 a chaeta (after Vejdovsky). c.e, Coelomic epithelium ; cm, circular 

 muscles ; ch, chaeta ; ec, epidermis ; l.m, longitudinal muscles ; m, 

 muscle for moving chaeta. [The cuticle is indicated by a thick 

 black line.] 



