^SS^ Dr. T. Williams on the Mechanism of Aquatic 



readily to and fro in his chamber, and to shield the soft segment 

 of the body from injury. It is not improbable that the layer of 

 water thus placed between the body of the mollusk and the shell, 

 may materially assist in the excretionary process as described by 

 Mr. Huxley, by which the latter is formed. It may add fluid 

 to, or dissolve the excreted material furnished by the surface of 

 the mantle, and adapt it to the internal surface of the shell. 

 The limited ligament (fig. 2 b) by which the animal is organic 

 colli/ united to its shell, places it beyond doubt that the latter 

 can be formed by no other process. 



By a few general observations, one department of the sub- 

 ject has been thus disposed of. No reference will hereafter be 

 made to this subject, namely to the relation which subsists be- 

 tween the abdominal segment of the body in Gasteropods and 

 external circumstances. It is extraordinary how in this class of 

 mollusks the most active forces of life are developed and spe- 

 cialized in the thoracic region, and how comparatively passive 

 are those parts which are bounded by the limits of the abdo- 

 men ! Circulation and respiration are functions which belong 

 to the former division. Large chylopoietic viscera are a resultant 

 phsenomenon. Thoracic development is a dynamic expression of 

 an organic power. Without it, other results could not follow. 

 It is the sign of power — without which the vegetative processes 

 could not be sustained. Between the Acephala and Cephalophora 

 there is this striking difference: — In the former the mouth is 

 placed in the respiratory cavity, in the latter it is the anal ori- 

 fice. In the former the alimentary and respiratory chambers are 

 confounded, in the latter the breathing and the cloacal cavities 

 are identified. This is a wide mark between the I^amellibranchs 

 and the Gasteropoda. ^i ^siosaa signia oa 



The higher the animal in the scale of life, 'fchie'Baore Vigorous 

 are the dynamic active powers. Thus, in the Gasteropod mol- 

 lusks as compared with the Acephala, the physiologist expects 

 an increment of vivifying force. How is it accomplished ? By 

 a more fully developed heart, impelled by the vital battery of a 

 more highly organized cephalic ganglion, by a. more actively 

 endowed thoracic apparatus, by respiratory movements of a 

 higher muscular character, and by branchial or pulmonary 

 systems of incomparably more intricate workmanship. 



In looking upwards along the line of the Cephalophora, it will 

 be seen that the head as a detached member, as a distinct clas- 

 sificatory character, appears long before the respiratory organs — 

 ex. Pteropoda, Heteropoda, Apneusta, and many Nudibranchs. 

 Though the Encephala discover several marked signs of supe- 

 riority as measured by the standard of the Acephala, it is well 



