82 



BALANOGLOSSIDA. 



10 



teriorly this septum ends with a free edge, the anterior part of the 

 proboscis cavity being single, but posteriorly it overlaps, though 

 it does not join, the dorsal and ventral septa above referred to. 

 In this overlapping part the muscle fibres of the dorso- ventral 

 muscle plate diverge on reaching the central complex and em- 

 brace that structure. 



The ventral septum consists of a layer of basement membrane 

 which is continuous with the basement membrane of the body 

 wall and with that surrounding the notochord (Fig. 64, vs ; 

 Fig. 65, 6 ; Fig. 67, sv). It ends with a free edge in front, rarely 



extending be- 

 yond the front 

 end of the central 

 complex, some- 

 times not so far. 

 Posteriorly it 

 either extends to 

 the very hind end 

 of the ventral 

 canals, com- 

 pletely separat- 

 ing them, or it 

 stops short of 

 this, ending with 

 a free edge, so 

 that the ventral 

 canals join pos- 

 teriorly (Fig. 66). 

 The basement 

 membrane of 

 the ventral sep- 

 tum at the point where it joins the basement membrane of 

 the ventral proboscis wall is pierced by the fibres of the circular 

 muscular layer. 



The dorsal septum, which separates the dorsal canals, is formed 

 by the walls of the pericardium, the basement membrane of 

 which is continuous with the basement membrane of the body 

 wall in the dorsal middle line (Fig. 65). It is usually continued 

 forwards as an effective septum by the fibres of the dorso-ventral 

 muscle plate (Fig. 64), which may overlap the anterior portion 



FIG. 65. Diagram of a transverse section through the proboscis 

 of a Balanoglossid (from Lang after Spengel). 1 dorsal septum ; 

 2 ectoderm ; 3 tlood vessels of the integument ; 4 circular, 

 5 longitudinal muscular layer ; 6 ventral septum ; 7 noto- 

 chord ; 8 coelom ; 9 heart ; 10 glomerulus ; 11 pericardium. 



