34 



G. 0. SARS. 



[NO. 1. 



below the umbones. This movement is rather powerful, and gen- 

 erally takes place on the animal getting disturbed, accompanied 

 as it then invariably is by a more or less complete retraction of 

 the body within the shell (see Pl. II, fig. 1). 



Movements of the Head. — The head admits of being moved 

 in relation to the cervical division of the body in two directions, 

 both movements apparently effected by muscular action. In the 

 one case, it is elevated so as to approach the dorsal part of the 

 shell, thus exserting its anterior part in front of the latter, an 

 attitude constantly retained during the swimming motion. In 

 the other case, the head is moved down so as to form a more 

 or less sharp curve with the succeeding part of the body, 

 being at the same time completely withdrawn into the shell 

 (see Pl. II, fig. 1). The first of these movements is very slow, 

 and apparently effected by the anterior pair of the two diverging 

 muscles running from the upper part of the shell across the 

 dorsal prolongation of the cervical division. The contrary 

 movement, on the other hand, is rather rapid and energetic, 

 effected in all probability by the anterior part of the two 

 ventral muscles of the trunk joining the head at the oral 

 region. 



Movements of the Trunk. — This division of the body is 

 rather mobile and, together with the tail, admits of being freely 

 swung to a very great extent within the shell. The movements 

 are chiefly a flexion and an extension, besides perhaps a slight 

 elevation, the latter apparently effected by the aid of the pos- 

 terior pair of the above-mentioned muscles running from the upper 

 part of the shell to the body. The flexion of the trunk, performed 

 by the two slender ventral muscles described above, is often 

 so considerably that this part forms an almost semicircular 

 curve, whereby the caudal plate is brought in close approxima- 

 tion to the oral parts. The extension of the trunk, effected 

 by the strong dorsal muscles, is generally much more power- 

 ful and rapid than the flexion, and causes this part to be 

 nearly straight, the caudal plate being projected beyond the pos- 

 terior part of the shell (see Pl. III, fig. 1). These movements 



