THE CEANIAL NERVES AND LATERAL SENSE ORGANS OF FISHES. 191 



" are transformed from the canal type during ontogeny," and concludes that " the 

 Torpedoes possess a system of organs which is lacking in all other Batoids, but which 

 is represented in these forms by the more primitive canal type." It cannot, I think, 

 be admitted that there is any foundation for the view that Savi's vesicles are deirenerate 

 canal organs. The observations of Allis, Coggi, Boll, Merkel, and numerous other 

 authors, show that the pit organs, Savian vesicles, and other more or less superficial sense 

 organs, develop as the canal organs do up to a certain stage, but that there they stop. 

 There is no evidence of retrogression, and none in fact that any of the lateral line organs 

 ever acquire a more central position than that which they ultimately occupy in the adult. 

 The fact that a sensory canal of one form may be represented by a line of pit organs 

 in another, may be more reasonably explained by precisely the opposite hypothesis, 

 i. e. that the one type represents a more advanced condition than the other. This is 

 the view adopted by most morphologists, and the one that has the sanction of 

 the facts *. 



The pit organs oi Amia are correctly homologised by E wart & Mitchell (1892, 69) 

 Avith the sensory follicles they described in the Skate; and the '' Spalt-papillen " of 

 Fritsch. Bashford Dean (1895, 58), in referring to the pbylogeny of the lateral canals, says 

 (p. 49) : — " The ancestral condition of the lateral line of Sharks appears to have been 

 represented in an open continuous groove, lined with ciliated sense cells, and protected 

 only by an overlapping margin of sliagreen denticles. In this condition it at least exists 

 in the ancient Sharks and in CJiimcera. That the canals of the head rea-ion were also 

 primitively of this character appears cvceedingly probable; they are thus retained in 

 the adult Chumera." Ley dig (1895, 128) gives a brief expression to a very important 

 and significant fact when he states that there are so many varieties of lateral sense 

 organs, from the superficial organs to the canal organs, that it is impossible either to 

 divide them into groups or even to draw sharp lines between individual forms. 



A passage in Miss Platfs work on Nectariis {I'fi^Q, 158) is interesting in this con- 

 nection. She says (p. 526) : — " At each side of a mid-dorsal fold in the skin a row of 

 mucous glands is found, composed of a few cells invaginated from the deeper layer of the 

 ectoderm, and now lying below the surface, tiny balls of cells surrounding a central 

 cavity that opens to the surface by a small pore. Similar glands are found on the 

 ventral surface of the body between the fore-limbs, and on the tail. Although these 

 glands are about the size of sense organs, nothing in their structure or in the manner of 

 their development suggests the genetic affinity of sense organ and mucous gland on 

 which Leydig [1895, 128] insists." Whilst it is true, as Miss Piatt maintains, that 

 there is no genetic affinity l)et\veen the lateral sense organs and mucous glands, it is also 

 true that a lateral sense organ consists of sensory and secretory portions, and it is indeed 

 probable, as Eisig suggests, that certain dermal mucous organs have become incorporated 

 into the lateral line system. In his last Amia paper (1897, 6) Allis says (p. 627) : — 

 " In early stages of development they [i. e. canal organs and pit organs] closely resemble 



* Ayers' views on the morphology of the semicircul;i,r canals compel him to take up the position that he does 

 (see particularly pp. 218-220;. Nevertheless I submit that tJicre is no Icuowii fact in the Jdstori/ of the lateral Jim 

 system tending to sJiow that there is any differentiation in the canals after thsy are once enclossd. 



