134 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



epithelium, lodged in the foot, and opening to the exterior by a 

 small pore beneath the mouth. 



(3) Auditory Organs. A minute spherical otocijst or auditory 

 sac is placed on each side of the ventral nerve-mass, and is con- 

 nected by an auditory nerve with the corresponding cerebral 

 ganglion. The sac possesses an outer firm investment, and is 

 lined by ciliated columnar epithelium, composed of auditory cells. 

 It is filled with numerous calcareous particles or otolitlis, suspended 

 in fluid. 



(4) Visual Organs. Each optic tentacle bears on the outer side 

 of its tip an eye, which appears as a black dot, and is innervated by 

 the optic nerve. It is placed immediately below a small transparent 

 area of the epidermis, and essentially consists of a spherical vesicle, 

 enclosed in a firm sheath, and containing a large globular lens, 

 devoid of structure. The vesicle is made up of a single layer of 

 cells, which in front are short and transparent, constituting a 

 cornea, while behind they are much more elongated and form a 

 sensitive retina. The retinal cells are of two kinds, unpigmented 

 and pigmented. Each of the former is produced into a flask- 

 shaped visual rod next the cavity of the vesicle, and is surrounded 

 by several of the latter, which are pigmented in their outer parts 

 only, while internally they taper to transparent processes ensheath- 

 ing the visual rod. 



It has been shown that the eyes of the snail can only clearly 

 distinguish the form of external objects when at a distance of from 

 A to JV of an inch- The pigmented skin appears to be sensitive 

 to light, helping the animal to distinguish between light and 

 darkness. 



CHAPTER VIII. VERTEBRATA ACRANIA. 

 13. AMPHIOXUS (The Lancelet). 



AMPHIOXUS is a semi-transparent somewhat fish-like animal, not 

 exceeding two inches in length, and laterally flattened. There is 

 a free-swimming larva, but the adult animal is a shallow water 

 marine form generally found vertically buried in the sand, from 

 which only its anterior end projects, but also capable of swimming: 

 by eel-like movements of its body. It is abundant round many 



