418 



INTRODUCTION TO CRANIATA 



[CH. 



may also receive impulses from the terminal tufts of several axons, 

 and in this way impulses are co-ordinated and combined. 



As mentioned above, the skull and brain are by no means the 

 only characters which distinguish the Craniata from other Chordata. 

 Perhaps the next in importance is the possession of three well- 

 developed pairs of sense-organs, nose, eyes. and ears. 



or S !! e ~ Of tnese *ke nose * s tne most sim Ply constructed. 



It consists merely of a pair of pits in the skin at the 



most anterior portion of the body, the lining of which develops 



ridges covered with sensory cells, having an olfactory function 



Fm. 208. Transverse section through the snout of a Dogfish, Scyllium canicula, 

 to show the structure of the nose x 2. 



I. Opening of olfactory sac. 2. Olfactory epithelium. 3. Ethmoidal 

 region of the cartilaginous skull. 



(Fig. 208). The essential element in all sense-organs is the sense- 

 cell, which generally resembles the neuron in possessing a basal 

 process terminating in a tuft of dendrites by which the stimulus is 

 transmitted as an impulse through a neuron, for in Craniata a 

 sense-cell is never in direct communication with a muscle-fibre. 

 Where the basal process is absent the body of the sense-cell itself is 

 enwrapped by the branching receptive dendrites of a neuron beneath 

 it. An olfactory sense-cell differs from a neuron in possessing one 

 or more stiff peripheral processes projecting from the surface of the 

 body, by which stimuli are received from the external world. These 

 are termed sense-hairs, and they are excessively delicate in struc- 

 ture. Sense-cells are never combined by themselves into an epi- 

 thelium : they are always intermixed with stiff supporting cells 



