MOLLUSCA. 267 



tween tlienij in a horizontal direction ; and by this means the 

 Snail can be made to undulate to and fro, obeying exactly 

 the movements of the rod. This requires to be done very 

 gentlj^, as if too much force is used the web is broken and 

 the Snail rises rapidly to the surface of the water." 



AsciDiA viTREA (?). — (Plate XIII.) 



This is a shell-less Mollusc of the Tunicate Order, appa- 

 rently as simple in form as a Poli/pe. The beautiful group 

 which I have attempted to portray in my Plate was brought 

 from Ilfracombe. Eight or i^n specimens have nestled in 

 a bunch of P/iT/llophora rubra. They consist of a bottle- 

 shaped sac with two necks and mouths. They are jelly-like 

 and transparent, and their open mouths are scalloped, with 

 a little marking of red and opaque-white at each notch. 

 Scarcely any motion is to be observed, excepting every now 

 and then either the large opening or mouth, or the smaller 

 one, suddenly shut sup, soon to be opened again. Some- 

 times an intruding substance is thrown out with force, and 

 the apertures kept closed for some time after. 



NuDiBRAXCHiATE MoLLUSCA. — (Plate XIV.) 

 The ^ahed-fjilled Mollusca have no shells, and their gills 



