GENERAL CHARACTERS 271 



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

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

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

 (sep\ 



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 (./?. V) 



Nephridia (Nphm) 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 

 ((Es. 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. 

 Epthm), muscle-plates (M. PI), and parietal layer of ccelomic epithe- 

 lium (Ccel. Epthm). 



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

 covered by the visceral layer of ccelomic epithelium (Gael. Epthm) : in 

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

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

 oesophagus ; int, intestine ; ret, rectum. 



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

 epithelium. 



Two 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 (Cu), deric epithelium (Der. Epthm), and parietal 

 layer of ccelomic epithelium (Ccel. Epthm}, forming the body-wall ; and 

 the enteric epithelium (Ent. Epthm) and visceral layer of ccelomic 

 epithelium (Ccel. Epthm 1 ) forming the enteric canal. 



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

 formed of a double layer of coelomic 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 (deric 

 epithelium) is well shown. 



Fig. 72, A (p. 290), should be compared with this figure, as it is an 

 accurate representation of the parts here shown diagrammatically. 



