Evolution of Animals 427 



has been traced, but it may be that chemotactic and olfactory 

 functions are conjoined in the disk area, and only become 

 separated as recognizable sense-organs in higher groups. But 

 Zelinka has described {11^0: 4 '24) a median anterior mass of 

 four cells in front of the developing brain in embryonic Calli- 

 dintty that becomes in the adult animal a median sensory de- 

 pression with sensory bristles, and which he terms the "taster." 

 It receives a median anterior nerve from the brain, and is the 

 first sense organ to differentiate. So alike in time of origin, 

 in position, structure, and nerve supply it agrees with a median 

 olfactory organ. 



When we compare this with the median ciliated center in 

 some turbellarians and nemerteans, with the median ciliated 

 olfactory sac or sacs in cyclostome fishes, and with the minute 

 studies of Herrick and Johnston {162 ^ 163) on the origin and 

 relation to the evolving cerebrum of the olfactory apparatus 

 in vertebrates, a direct morphological and physiological con- 

 tinuity is suggested that may yield valuable results when 

 exhaustively pursued. 



The eyes may be 2, 3, or 4 in number, may either remain 

 separate or be fused in pairs, and they receive paired nerves 

 from the brain. In some genera, two minute openings lead 

 into sacs containing refractive particles, and these, in position, 

 nerve-supply, and connections, suggest the geotactic portion 

 of the vertebrate ear. One to two pairs of antennae that may 

 remain distinct, or that may fuse more or less, are referred 

 to in another chapter (p. 531). Except in VorticeroSy these 

 organs seem to be absorbed in rhabdocoelids, as the evolving 

 tactile proboscis of that group takes their place mth increas- 

 ing efficiency. 



In rhabdocoel turbellarians certain of the epidermal cells 

 produce varied secretions. Such glandular cells are grouped 

 as mucus, pigment, and hyaloid, according to the substance 

 secreted, or the aspect of the contents. 



The special sense-organs are rarely tentacular as in Vorti- 

 ceroSy since the progressive formation of a proboscis evidently 

 causes a concentration of the tactile function in the latter. 



In several fresh-water rhabdocoel genera a median orifice 

 in front of the mouth, and close to the cephalic ganglia, leads 

 into a cavity that may become bilobed, is richly ciliate over 

 its anterior area, and has a nerve-ending in it. This we would 

 regard as the rudiment of the olfactory organ, though more 

 advanced than in Rotifera, that leads up to the corresponding 

 structure seen in nemerteans, and is there called the frontal 

 organ. In other wholly or mainly fresh-water genera, paired 



