104 Lh'ITISH TUXICATA. 



wedge-shaped process projects from the inner surface 

 of the right side, and, penetrating into the loop of the 

 intestine, holds the visceral mass firmly in its position. 

 This process does not merely enter the loop, but is 

 attached to the walls of the alimentary tube throuo-h 



v O 



the agency of the mantle (PL IX, fig. 1), which is a 

 delicate membrane but sparingly supplied with mus- 

 cular fibres. 



The branchial sac (PI. X) is more delicate than 

 usual and is minutely plicated ; the plica? however in 

 some of the specimens from Shetland are rather 

 obscure and difficult to observe. The primary vessels 

 are regularly arranged and pretty equal in size, a few 

 only being a little larger than the rest ; the secondary 

 vessels are rather longer than usual and the stomata 

 are widish, with the extremities somewhat pointed. 

 The papillae at the intersections of the longitudinal 

 bars are stout, conical, and obtuse, with occasionally a 

 few scattered, small, intermediate ones ; the papillary 

 membrane is strongly developed. The oral lamina (PL 

 IX, fig. 2) is wide, with the right side rather closely 

 ribbed ; it reaches to the bottom of the sac and retains 

 its width considerably below the mouth ; a narrow 

 lamina with denticulated margin extends from a little 

 above the mouth on the left side to the lower ex- 

 tremity of the sac. The branchial tubercle (PL IX, 

 fig. 2) is very minute and horse-shoe-like with the arch 

 downwards. The tentacular filaments, of which there 

 are between sixty and seventy, are arranged in a 

 single line ; they are long and slender, but vary con- 

 siderably in length. 



The ovary (PL IX, fig. 4) is a branched tubular 

 organ ramifying over the left side of the intestinal loop, 

 covering the whole of this portion of the alimentary 

 tube with an open network of thickish contorted 

 branches. The testis (PL IX, fig. 4) is largely de- 

 veloped, and is spread over the entire left side of the 

 stomach and part of the intestine, covering a consider- 

 able portion of the other side of the alimentary tube 



