538 ORGANS OF THE LATERAL LINE. 



of forming the posterior nares soon vanishes, and by the growth of the front 

 of the head the nasal pits are carried farther away from the mouth. 



The actual posterior nares are formed by a perforation in the palate, 

 opening into the blind end of the original nasal pit. 



Considering that the various stages in the formation of the posterior nares 

 of the Amniota are so many repetitions of the adult states of lower forms, it 

 may probably be assumed that the mode of formation of the posterior nares 

 in Amphibia is secondary, as compared with that in the Amniota. 



A diverticulum of the front part of the nasal cavity of the Anura is 

 probably to be regarded as a rudimentary form of Jacobson's organ. 



BIBLIOGRAPHY. 



(394) G. Born. "Die Nasenhohlen u. d. Thranennasengang d. amnioten 

 Wirbelthiere." Parts I. and n. Morphologisches Jahrbuch, Bd. V., 1879. 



(395) A. Kb'llicker. " Ueber die Jacobson'schen Organe des Menschen." 

 Festschrift f. Rienecker, 1877. 



(396) A. M. Marshall. "Morphology of the Vertebrate Olfactory Organ." 

 Quart. Journ. of Micr. Science, Vol. XIX., 1879. 



Sense organs of the lateral line. 



Although I do not propose dealing with the general development of 

 various sense organs of the skin, there is one set of organs, viz. that of the 

 lateral line, which, both from its wide extension amongst the Ichthyopsida 

 and from the similarity of some of its parts to certain organs found amongst 

 the Chaetopoda 1 , has a great morphological importance. 



The organs of the lateral line consist as a rule of canals, partly situated 

 in the head, and partly in the trunk. These canals open at intervals on 

 the surface, and their walls contain a series of nerve-endings. The 

 branches of the canal in the head are innervated for the most part by 

 the fifth pair, and those of the trunk by the nervus lateralis of the vagus 

 nerve. There is typically but a single canal in the trunk, the openings 

 and nerve-endings of which are segmentally arranged. 



Two types of development of these organs have been found. One of 

 these is characteristic of Teleostei ; the other of Elasmobranchii. 



In just hatched Teleostei, Schulze (No. 402) found that instead of the 

 normal canals there was present a series of sense bulbs, projecting freely 

 on the surface and partly composed of cells with stiff hairs. In most 



1 The organs which resemble those of the lateral line are the remarkable sense 

 organs found by Eisig in the Capitellidse (Mittheil. a. d. Zool. Station z# Neafcl, 

 Vol. I.) ; but I am not inclined to think that there is a true homology between these 

 organs and the lateral line of Vertebrata. It seems to me probable that the 

 segmentally arranged optic organs of Polyophthalmus are a special modification of the 

 more indifferent sense organs of the Capitellidse. The close affinity of these two 

 types of Chsetopods is favourable to this view. 



