272 POLYGORDIUS less. 



Between the enteric canal and the body-wall is the coelome (Qr/), 

 divided into right and left portions by the dorsal {D. Mes) and vential 

 ( v. Mes) mesenteries, and into segmental compartments by the septa 

 {Sept). 



Lying in the mesenteries are the dorsal {D. V) and ventral (V. V) 

 blood-vessels, connected by commissural vessels {Com. V) running in 

 the septa : from the latter go otl' recurrent vessels {H. V). 



Nephridia {Nphm) are shown in the second and third metameres, 

 each consisting of a horizontal jwrtion which perforates a septum and 

 opens in the preceding segment by a nephrostome {Nph. st) and of a 

 vertical portion which perforates the body-wall and opens externally by 

 a nephridiopore {Nph. p). 



The bram {Br) lies in the prostomium and is connected with the 

 ventral-nerve cord {V. Nv. Cd.) by a pair of oesophageal connectives 

 {CEs. Com). 



B, diagrammatic longitudinal section showing the cell-layers. 



The cuticle is represented by a black line, the ectoderm is dotted, 

 the endoderm radially striated, the muscle plates evenly shaded, the 

 ccelomic epithelium represented by a beaded line, and the nervous 

 system finely dotted. 



The body-wall is composed of cuticle (f«), deric epithelium {Der. 

 Epthm), muscle-plates {AI. PI), and parietal layer of ccelomic epithe- 

 lium {Ctvl. Epthm). 



The enteric canal is formed of enteric epithelium {Ent. Ei>thm) 

 covered by the visceral layer of coelomic epithelium (Ccv/. Epllim') ; in 

 the neighbourhood of the mouth (yl/Mjand anus(.^«) the enteric epithe- 

 lium is ectodermal ; elsewhere it is endodermal ; Ph, pharynx ; Oes, 

 oesophagus ; /«/, intestine ; Ret, rectum. 



The septa {Sept) are formed of muscle covered on both sides by coelomic 

 epithelium. 



Four nephridia {Nphm) with nephrostome {Nph. st) and nephridiopore 

 (Nph. p) are shown. 



The brain {Br) and ventral nerve cord ( V. Nv. Cd) are seen to be in 

 contact with the ectoderm : from the brain a nerve {nv) passes to the 

 tentacle. 



C, diagrammatic transverse section showing the cell-layers as in B, 

 viz. the cuticle (C«), deric epithelium (Der. Epthm), muscle-plates 

 {M. II), and parietal layer of coelomic epithelium {Cat. Epthm), form- 

 ing the body-wall ; and the enteric epithelium {Ent. Epthm) and 

 visceral layer of coelomic epithelium {Cat. Epthm'), forming the enteric 

 canal. , 



The dorsal {D.Mes) and ventral {V. Mes) mesenteries are seen to be 

 formed of a double layer of ccelomic epithelium, and to enclose respec- 

 tively the dorsal {D. V) and ventral ( V. V) blood-vessels. 



A nephridium {Ni>hvi) is shown on each side with nephrostome {Nph. 

 st) and nephridiopore {Nph.p). 



The connection of the ventral nerve- cord with the ectoderm (deric 

 epithelium) is well shown. 



Fig. 70, A (p. 291), should be compared with this figure, as it 

 is an accurate representation of the parts shown here diagram- 

 matically. 



