37 POLYGORDIUS LESS. 



Between the enteric canal and the body-wall is the ccelome (Co?/), 

 divided into right and left portions by the dorsal (D. Mes) and ventral 

 ( V. Mes] mesenteries, and into segmental compartments by the septa 

 (Sept). 



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

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

 the septa : from the latter go off recurrent vessels (R. V}. 



Nephridia (Nphni) are shown in the second and third metameres, 

 each consisting of a horizontal portion 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 brain (Br) lies in the prostomium and is connected with the 

 ventral -nerve cord (V. Nv. Cd.) by a pair of cesophageal 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 (Cu), deric epithelium (Der. 

 Epth/n), muscle-plates (M. PI], and parietal layer of ccelomic epithe- 

 lium (Ca'l. Epthm}. 



The enteric canal is formed of enteric epithelium (Ent. E-bthni) 

 covered by the visceral layer of coelomic epithelium (Cccl. Epthm') ; in 

 the neighbourhood of the mouth (.#///) and anus(^4) the enteric epithe- 

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

 oesophagus ; Int, intestine ; Ret, rectum. 



The septa (Sept) are formed of muscle covered on both sides by ccelomic 

 epithelium. 



Four nephridia (Nphni) 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 c'uticle (Cu), deric epithelium (Der. Eptht/i), muscle-plates 

 (M. //), and parietal layer of ccelomic epithelium (Coel. Epthm), form- 

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

 visceral layer of ccelomic epithelium (CceL Epthni), 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 (Nphni) 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. 



