MAX. 321 



Rabbit possess nasal sacs largely lined by olfactory epithelium 

 containing numerous olfactory cells. 



(d) Auditory Organs. These are first recognizable in the Cray- 

 fish, where they exist in the form of small, open, auditory sacs, 

 lodged in the basal joints of the antennules, and lined by sensory 

 epithelium. In the Mussel and Snail closed sacs are found, similar, 

 in essential respects, to the open sacs of the Crayfish. By the 

 growth of such simple vesicles into a complicated shape, the 

 membranous labyrinths found in the Dogfish, Frog, Pigeon, and 

 Rabbit are formed during development. To these may be super- 

 added accessory parts making up middle and external ears. 



The function of the " auditory organs " of Invertebrates is ill understood. 

 In some cases they have been shown to be concerned with the perception of 

 position in space, as is the case with the semicircular canals of Vertebrates. 



(e) Visual Organs. Two eyes in the form of pigment-spots are 

 possessed by the ciliated embryo of the Fluke. In the Earthworm 

 the cerebral ganglia themselves appear to be sensitive to light 

 acting upon them through the translucent skin; and the pigmented 

 part of the skin is sensitive to light (as also in the Snail). The 

 Leech possesses eyes, which appear to be modified tactile organs. 

 The Crayfish possesses compound eyes, to which the theory of 

 "mosaic vision" is applicable, while the simple -eyes of the Snail 

 resemble in principle those of the Frog, Pigeon, and Rabbit, which 

 have a double origin, being partly formed by involutions from 

 the exterior, and partly by outgrowths from the fore-brain. It 

 is doubtful whether the so-called " eye " of Amphioxus has a 

 visual function. 



CHAPTER XIV. MAN. 



THE aim of this chapter is to form a connecting link between the 

 subjects of General Biology and Human Anatomy as successively 

 taken in a medical course, and it mainly consists of a statement 

 of the leading differences between Rabbit and Man, so that a 

 student who has carefully dissected the former will be able to 

 turn the knowledge thus gained to useful account when he comes 

 to dissect the latter. Comparative Anatomy is also of import- 

 ance in relation to therapeutics and pathology. Those characters 

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