148 EDWARD PHELPS ALLIS, JR. 



edly similar innervations, and transverse sections of the two or- 

 gans, as shown by Herrick ('03) and Kistler ('06), are somewhat 

 similar. The nerve pits of Polyodon are found in little groups, 

 and certain of the pits in each group are arranged in little clus- 

 ters, the group apparently having been formed, as suggested in 

 my work on the lateral canals of this fish (Allis, '03), by the com- 

 plete subdivision of a single primitive pit into several independ- 

 ent pits, and the subsequent incomplete subdivision of these 

 secondary pits to form the little clusters above referred to. 

 If one of these little clusters were to sink beneath the surface, it 

 would apparently give rise to a nerve sac such as is found in 

 Acipenser, for Merkel ('80) says that a large proportion of these 

 nerve sacs open on the external surface by a number of pores 

 lying close together. These nerve sacs must closely resemble 

 the ampullae of Chlamydoselachus, for these ampullae are, as 

 Merritt Hawkes ('06) has stated, pear-shaped structures which 

 lie immediately beneath the dermis and are formed by little 

 packets of from two to six simple ampullae, all of these simple 

 ampullae having short tubules which open independently, 

 but close together, at the bottom of a slight depression on the 

 outer surface of the skin. 



It would therefore seem quite certain that the nerve pits of 

 Polyodon are a primitive form both of the nerve sacs of. Aci- 

 penser and the ampullae of the Selachii, and it is for this reason 

 that I have called them nerve pits, instead of either pit organs 

 or primitive pores. It would also seem as if these nerve pits, 

 and hence also the Selachian ampullae, must be represented, in 

 the Teleostei, by the small pit organs of Herrick's descriptions, 

 and the latter author has already suggested this homology for 

 the ampullae ('03, p. 135). This being so, it would be natural 

 to assume that the nerve pits and small pit organs should re- 

 spond to the same stimulus as the ampullae, which, according 

 to Metcalf ('15), is that of pressure, the function of the ampullae 

 being to inform the fish of the depth of the water in which it is 

 swimining, the direction of currents, and possibly also of the 

 presence of vibrations of low frequency ('deeper notes'). The 

 habits and habitat of Polyodon are, however, opposed to this 

 assumption. 



