46 Dr. T. Williams on the Mechanism of Aquatic 



those of the mantle of the higher Acephala ; and what differen- 

 tiation of these parts was required to enable an ordinary structure 

 to discharge a special office, has been shown by Dr. Carpenter. 



The organs of breathing are well developed in all the Lamel- 

 libranchiate Acephalans. Their vascular system is elaborately 

 multiplied. They are capable of containing a considerable amount 

 of blood. If aquatic were not less intense than atmospheric re- 

 spiration, the aggregate area of the surface exposed by the gills 

 of mollusks in general would insure a measure of effect sufficient 

 to raise these animals high in the scale of physiological activity. 

 Surface is not the only factor to be counted in determining the 

 dynamic value of the respiratory office. The composition of the 

 blood demands a numeric place in the calculation. If the fluid 

 occupying the vessels were identical in density with the exterior 

 element, no interchange of gases could proceed. A difference 

 in the specific densities of the gases held in solution by fluids of 

 identical gravities would constitute a condition in virtue of which 

 the gases would reciprocally move independently of the fluids. 

 The less the proportion of fibrine in the blood, other things being- 

 equal, the lower is its absorptive capacity for gases. The blood 

 of mollusks is less charged with fibrine than that of the higher 

 Articulata. In the former the floating corpuscles are less highly 

 organized. They are strikingly less filled with solid contents. 

 They are smaller and yet not more numerous. The physical 

 conditions as regards the fluids then are not favourable in the 

 Mollusca to a high rate of respiration. * 



Cuvier first defined the bivalve mollusks under the title of 

 Acephala testacea. By Lamarck they were constituted into a 

 separate class under the name of Conchifera. M. de Blainville 

 marshalled them under the order Acephalophora lamellibran- 

 chiata. The anatomical definition of Cuvier presents clearly 

 the chief points of structure : — " Leur corps qui renferme le foie 

 et les visceres est place entre les deux lames du manteau ; en 

 avant, toujours entre ces lames, sont les quatre feuillets bran- 

 chiaux stries regulierement en travers par les vaisseaux ; la bouche 

 est a une extremite, l'anus a fautre, le coeur du cote du dos ; le 

 pied, lorsqu'il existe, est attache entre les quatre branchies*/' 



The mantle of the mollusk is a grand feature of the organism. 

 Its horizontal lobes embrace, its vertical process, on which the 

 branchiae are evolved structurally and functionally, bisects, the 

 whole body. The mantle at once invests and secretes the shell, 

 and forms the very basis of the body of the animal. It is com- 

 posed of muscles, nerves, fibres, and vessels. It is lined inter- 

 nally in all cases with vibratile epithelium. A straight line, 



* Reprip Animal, vol. sur les Mollusques, p. 182. 



