Royal Society. 297 



vigorous than that of the nervous centre and nerve, and far less 

 dependent upon the supply of blood, is immediately re-established, 

 and being re-established, the muscle relaxes (just as it does in 

 the case where paralysis is caused by division of the nerve), and 

 tremor, convulsion and spasm are at an end. Nor is there any doubt 

 that the nerves are paralysed when the circulation fails to the point 

 which is here supposed. Thus, if the circulation in the hand be 

 depressed by immersion in cold water, the sense of touch and the 

 power of movement are partially or wholly destroyed ; or if the 

 principal vessel of a limb be tied, the nerves are similarly paralysed 

 until the collateral circulation be established ; and in each case, also, 

 the power of provoking "reflex movements" is diminished or de- 

 stroyed. In either case the nerves are more or less paralysed for 

 want of blood, and, if so, it surely follows that the nerves must be 

 paralysed, and still more effectually, when the circulation fails as it 

 fails in syncope, asphyxia, or death, and when the movement of the 

 blood is almost or altogether at an end. Hence it is quite intelli- 

 gible that tremor, convulsion or spasm should be caused by want of 

 blood, as is stated in the argument, and that they should cease in 

 syncope, asphyxia, and death ; and thus this objection falls to the 

 ground, and with it all objections of the same kind. 



Such is an imperfect sketch of the evidence upon which the phy- 

 sical theory of muscular contraction is founded. 



Dec. 21, 1854. — The Lord Wrottesley, President, in the Chair. 



The following communication was read : — 



"Remarks on the Anatomy of the Macgillivrayia pelagica and 

 Cheletropis Huxleyi (Forbes) ; suggesting the establishment of a new 

 genus of Gasteropoda." By John D. Macdonald, R.N., Assistant- 

 Surgeon H.M.S. Herald. 



Having examined the anatomy of the Macgillivrayia pelagica and 

 several smaller species of pelagic Gasteropoda, not exhibiting the 

 least similarity in the character of their shells, the author found that 

 they all presented a very close relationship to each other in the type 

 of their respiratory organs, and in other points of structure of less 

 importance. 



The gills in every instance seemed to be fixed to the body of the 

 animal immediately behind the head, and did not appear to be ap- 

 pended to the mantle, as in the Pectinibranchiata properly so called. 

 They were also invariably four in number, and arranged in a cruci- 

 form manner round a central point. They were free in the rest of their 

 extent, elongated and flattened in form, with a pointed extremity, 

 and fringed with long flowing cilia, set in a frilled border. They 

 were, moreover, furnished with muscular fibres, both transverse and 

 longitudinal, and exhibited great mobility when protruded, but lay 

 side by side in the last whorl of the shell when retracted. 



The auditory capsules, each containing a spherical otolithe, were 

 closely applied to the inner and posterior part of the larger or ante- 

 rior ganglion of the subcesophageal mass. 



There were two tentacula, each bearing at the outer side of 



