PELONAIA. 141 



lying below it. The sac, which is much constricted 

 above, widens gently downwards, and is a little pro- 

 duced at the base, towards the ventral margin, where 

 the oesophagus enters it. It is composed of a simple, 

 rectangular reticulation, in which the primary trans- 

 verse vessels are regularly disposed, with smaller 

 transverse channels between them. The meshes are 

 parallelogrammatic, wide and rounded at the ends, 

 and crossed about the middle by the small transverse 

 vessels. The longitudinal cords are narrow ribbon- 

 like membranes, attached bv one edo-e to both the 



t/ o 



large and small primary vessels. 



. 



FIG. 81. Part of the branchial sac of Pelonaia corrugata. Much enlarged. 



The endostyle (PI. XL VII, figs. 2 and 4) is well 

 developed, and extends almost from one end of the 

 sac to the other, there being only a very short distance 

 between its lower end and the mouth. The oral 

 lamina (PI. XLVII, fig. 4) is a delicate, simple, smooth, 

 widish membrane, which terminates below at the right 

 side of the mouth, and above in the anterior cord 

 which connects it with the extremity of the endostyle. 

 The posterior cord is very short. The branchial 

 tubercle (PL XLVII, fig. 4; PI. XLVIII, fig. 18) has 

 the extremities well turned inwards, the convolutions 

 downwards, and tlie arched margin upwards. The 



