414 EVENINGS AT THE MICROSCOPE. 



S. mytilus. Its form is oblong, with rounded extremities, 

 the anterior obliquely dilated. This species affords a good 

 example of the various organs of locomotion. A trans- 

 parent oblong shield, which is quite soft and flexible, is 

 spread over the back, which does not prevent our eyes 

 discerning all the organs through it ; though, much more 

 commonly, the animal, when under the microscope, crawls, 

 belly upward, beneath the glass cover of the live-box. 

 Around the anterior part, which is broadened, are placed 

 cilia, which are vibratile ; these are continued round the 

 mouth, a sort of fold on the side. Towards the posterior 

 extremity on each side are other rows of cilia, which being- 

 large are well displayed. On the ventral surface, chiefly 

 towards the front part, are seen several thick pointed 

 processes, shaped like the prickles of a rose, but flexible, 

 and capable of being turned every way. These are the 

 ttncini, and are evidently used as feet, the tips being 

 applied to the glass. The optical effect of the throwing 

 about of these uncini, when the place which they touch 

 is in focus, is very curious. They are rapidly moved, but 

 without regularity ; the tips bend as they touch the 

 surface of the glass ; some of them seem to have accessory 

 hairs, equally long, but slender, proceeding from the 

 same base. On the hinder quarter of the ventral surface 

 are several thick pointed spines; these are inflexible, nearly 

 straight, placed side by side, but not in regular order, 

 some reaching beyond others. I have not seen these used, 

 but they commonly remain sticking out in a horizontal 

 direction. These organs are termed styles. Besides 

 these, there are three slender bristles, called setce, placed 

 at the hinder extremity, the central one in the line of the 

 body, the others radiating at an angle. These are dis- 

 tinguished from the cilia, not only by their length, but by 

 not being vibratile. The motions of these animals are 

 powerful, but, irregular and fitful, very much like those of 

 the former species. They dart hither and thither, back- 



