GENERAL PRINCIPLES OF ZOOLOGY. 



unstaining second part, the middle piece, and then the tail, a long 

 flagellum, which causes the active motility of the ripe sperma- 

 tozoon. Cytoplasm is usually present only in an extremely thin 

 layer surrounding the nucleus. 



The spermatozoa of nearly all animals, except the nematodes 

 and crustaceans, are constructed according to this type. In these 

 two groups it is worthy of notice that the spermatozoa are 

 remarkably large and incapable of motion, and that they enclose a 

 homogeneous strongly refractive body (fig. 36, k), previously not 

 found, the significance of which is not clear. The spermatozoon 

 of Ascaris (fig. 36, e) has the form of a sugar-loaf with a broad 

 rounded end, containing the nucleus; the spermatozoon of the 

 crayfish (fig. 36, y), on the other hand, has the shape of a cake- 

 pan, from whose periphery springs a circle of fine, stiff, and 

 pointed fibres. 



The two kinds of spermatozoa found in a few animals are problem- 

 atical. In the testis of one and the same individual of Paludina vivipara 

 occur together hair-like spermatozoa with corkscrew heads and vermiform 

 spermatozoa with a bunch of cilia on the hinder end. The first accomplish 

 fertilization ; the physiological significance of the second is unknown. 



The last modification of epithelium of which we have to speak 

 is sensory epithelium, characterized by the connexion of certain 



of its cells, the sensory cells, with 

 the finest twigs of branching nerves 

 which arise in the central nervous 

 system. This connexion may be of 

 two kinds. In the first the cell 

 (primary sense cell) is slender and 

 filiform, the position of the nucleus 

 being indicated by a swelling. The 

 peripheral end is concerned with 

 the reception of sensatory stimuli, 

 while the deeper end is continued 

 directly into the nerve ends and 

 correspondingly is branched into two 

 or more extremely fine processes 

 FIG. 37. Sensory epithelium, a, of an which take on the character of 



Act in ian ; /3, from the olfactory epi- ,*, -,! /n ow \ 



theiium of man; d, supporting cells; nerve nbrillae (ng. 67). In the 



second type the sensory nerve ends 



in a ganglion cell beneath the epithelium, which sends processes 

 into the latter, the ends of these being applied to the sensory cell 

 (secondary sense cell), the connexion being one of contact, not of 



