Metasoa Lumbricus. 243 



whilst the septa are merely plates serving for the attachment 

 and support of the nephridia and the alimentary system. 

 This large space, or rather series of spaces, is known as the 

 body cavity, or, better, ccelom. Glancing backwards at the 



FIG. 122. DIAGRAMMATIC TRANSVERSE SECTION THROUGH THE MIDDLE OF THE 

 LOLiY OF Liimbricus terrestris. (Milnes Marshall.) 



A, cuticle ; B, epidermis ; c, coelom ; CM, layer of circular muscles ; D, seta ; 

 DV, dorsal vesssl r E, cavity of intestine ; F, cavity of typhlosole filled wuh 

 hepatic cells ; G, epithelium of intestine ; H, hepatic cells clothing intestine ; 

 I, ventral nerve-cord ; J, nerve ; L, ciliated funnel of the nephridium ; LM, 

 layer of longitudinal muscles ; M, point where nephridium perforates the 

 septum ; N, o, p, loops of the nephridium ; R, external aperture of the 

 nephridium ; sv, subneural vessel ; v, lateral vessel ; vv, ventral vessel. 



structure of the types of the Hydrozoa discussed in the last 

 section we see that that space is entirely absent. The space 

 in the interior of Hydra or Obelia is not a coelom, but an 

 alimentary canal, or enteron, although one without an anus 

 and without differentiation into stomach and intestine. 1 



1 It is of the utmost importance that students should, from the very 

 commencement, guard against the misuse of the two terms ccelom, or 

 body-cavity, and enteron, or alimentary cavity. If a clear distinction 

 be not drawn between these two spaces, endless confusion is apt to be 

 created in the minds of beginners in the subject. The enteron may be 

 defined as a cavity always nutritive in function and enclosed by hypo- 



R 2 



