130 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



XIV 



has also become highly specialized in various directions. Much of it 

 forms packing or connective tissue and special tracts of this may become 



hardened and stiffened to constitute 

 the skeleton. Other tracts of it form 

 the blood-system. A large section of 

 the mesenchyme remains on the other 

 hand comparatively unspecialized as 

 a mobile defence force to the body. 

 Its cells retain the character of amoe- 

 bocytes, creeping about among the 

 tissues of the body, performing 

 scavenging and other important func- 

 tions and remaining permanently 

 mobilized, ready to concentrate and 

 attack alien organisms, such as 

 disease - producing microbes, which 

 have found their way into the body. 



LUMBRICUS 



The general features of the phylum 

 Annklida are conveniently studied 

 in a large earthworm of the genus 

 Lumbricus which, though not so 

 unspecialized as are some of the 

 marine annelids, has the advantage 

 of being almost everywhere obtain- 

 able and of being thoroughly suitable 

 for dissection. 



The general appearance of the 

 creature (Fig. 62) is familiar to every 

 one — rounded in section, pointed 

 towards the front or head end, flat- 

 tened from above downwards towards 

 the hinder end. The upper or dorsal 

 side is darker in colour, the lower 

 or ventral side is paler, and is 

 somewhat flattened. Sharply marked 

 circular grooves on the surface of the body divide it into a 

 large number of segments or somites. The general surface is covered 

 with a thin, translucent, somewhat iridescent, cuticle which is apt 



Fig. 62. 

 An Earthworm — Lum- 

 bricus. The left - hand 

 figure represents the dorsal 

 side : the right-hand figure 

 the ventral side (after 

 Graham Kerr, Primer of 

 Zoology). A, anus; CI, 

 clitellum ; dp, dorsal pore ; 

 M, mouth ; Ps, prestomium. 

 (5 , Male genital opening ; 

 XIV, fourteenth somite. 



"•■■A 



