xxv GENERAL CHARACTERS 275 



Between the enteric canal and the body-wall is the ccelome (Ccel}, 

 divided into right and left portions by the dorsal (D. Jfes} and ventral 

 ( V. JA's) 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 off recurrent vessels (R. V) 



Nephridia (Nphrn) 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. si), and of a 

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

 a nephridiopore (Nph. p}. 



The brain (JBr) 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}, cleric epithelium (Dcr. 

 Epthm}, muscle-plates (J/. PI}, and parietal layer of ccelomic epithe- 

 1 i u m ( Ccel. Epth in ) . 



The enteric canal is formed of enteric epithelium (Ent. Epthm] 

 covered by the visceral layer of coelomic epithelium (Ccel. Epthm!} ; in 

 the neighbourhood of the mouth (MtJi) and anus (An} the enteric epithe- 

 lium is ectodermal, elsewhere it is endoclermal ; Ph, pharynx ; Ocs, 

 oesophagus ; /;//, intestine ; Rcl, rectum. 



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

 epithelium. 



Four nephridia (Nph nt) 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 (Cu), cleric epithelium (Der. Epthm}, muscle-plates 

 (M. PL], and parietal layer of coelomic epithelium (Ccel. Epthm}, form- 

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

 visceral layer of coelomic epithelium (Ccel. 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 (Nphm} is shown on each side with nephrostome (Nph. 

 st} and nephridiopore Nph. p}. 



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

 epithelium) is well shown. 



Fig. 71, A (p. 294), should be compared with this figure, as it 

 is an accurate representation of the parts here shown diagram - 

 matically. 



T 2 



