MOLLUSCA. 317 



contracted themselves into a very small compass, as- 

 sumed a globular form, cast the tentacles from off their 

 branchial fins, lost their colour, and expired in a very 

 few moments. 



When handled, the white colouring matter of the 

 body is very easily detached, and leaves the correspond- 

 ing portions of the integument transparent; when a 

 considerable accumulation of air, in the form of small 

 bubbles, is visible amongst the viscera, and fully ac- 

 counts for the hydrostatic power the animal possesses. 

 Immediately behind the left most anterior branchial 

 fin there is a circular orifice or foramen on the side of 

 the body, and serving as a cloaca, through which I have 

 noticed, in some examples, the male organ protrude in 

 the form of a slender, white and contorted thread ; and 

 it may be questioned, whether the air contained in the 

 body of the mollusc be not received or expelled through 

 this orifice, as it is through a similar opening existing 

 in the water-snails. 



THE SEA LEECH. 



(Hirudo Muricata, Linn. Albione Muricata, Savigny. 

 Pentobdella Muricata, Leach .) 



More than twenty specimens of this annelidan were 

 taken from the mouth of a Tiger-shark * which we 

 captured in the Pacific, in lat. 29 N., long 168 W. 



In general appearance, structure, and average size, 

 they bear a close resemblance to our common medicinal 



* This is, I believe, an undescribed species of the Squalus family. 

 We captured the only specimen we saw ; and it was by far the largest 

 shark we noticed during the voyage. It measured twelve feet in length, 

 and the girth of body and expanse of jaws were in proportion. The 

 head is very broad and flat. The prevailing colour of the fish is dark- 

 gray ; the sides marked with vertical and oblique yellow stripes whence 

 the trivial name which sailors apply to this species. 



