THE CKAXIAL NEKVES AND LATEiiAL SEN«E OEGANS OE EiSHES. iHt) 



linear sense streak was the primitive structure from \ylucli all the modifications of the 

 lateral line took their origin, and that tlie segmentally arranged sense bulbs of Teleostei 

 are secondary dilfcreutiatious of this primitive structure." Wright (1884, 227), who 

 insisted on the liistological difference between the terminal buds of Merkel and the 

 lateral sense organs of Amiurus, re-discovers the pit organs of Emery and Merkel *, and 

 calls them the " accessory lateral organs," whilst Eisig considers the mucous sacs of 

 Myxine to be very ancient structures and homologises them witli the lateral mucous 

 organs of Annelids. He further thinks it probable tliat tlie latter have been incorporated 

 or bound up with the vertebrate lateral line organs. 



The cousins Sarasin, who believed that all the cells of a lateral line organ were formed 

 by the division of a single cell, announce the interesting discovery of two accessory 

 sense organs in Ichthyoplus (1887, 177J. One of these organs is a flask shaped structure 

 containmg a club like " cupola," and which is called the " Nebenohr " or " Becherorgan." 

 An examination of the description of this structure at once suggests the probability that 

 it corresponds to a Lorenzini's ampulla of a cartilaginous fish. Tlie other sense organ 

 is a more superficial structure, and this may be said to represent the fish pit organ as 

 described by Emery, Allis, and Ramsay Wright. The peculiarity about the various 

 species of more or less superficial sense organs found in Fishes and Amphibia is that 

 whilst they are sufficiently similar in structure to denote a genetic affinity, yet the 

 corresponding organs in different types never precisely agree. 



Eritscli and Garmau were, as far as I am aware, the tirst authors to state that the 

 pit organ class of sense organ belonged definitely to the lateral line systeui. Earlier 

 authors, as already stated, had agreed in ranking together the three classes of integu- 

 mental organs as sensory and not mucus secreting structures, but hitherto no one had 

 recognised that ISavi's vesicles, pit organs, &c., Lorenzini's ampuUaj, and the lateral 

 canals themselves, all belonged to one and the same system. Eritsch's works 

 (1888, 76, & 18'JU, 77j contain the tirst description of the " Spalt-papillen " of Raia 

 previously referred to, and wliich I homoiogised with the pit organs and Savian vesicles 

 of other forms, whilst Garman not only recognised the connection of the pit organs 

 with the lateral line system, but also rightly saw in them organs in an intermediate or 

 transitory condition. He considers in tact that the pit organs are on their way to 

 becoming canal organs, but his assumption that they are consequently f uuctionless is a 

 physiological anachronism which is somewhat out of the question. 



In his tirst Amia paper, Allis (188!), 4) describes the development of the sensory canals 

 as occurring in the manner foreshadowed by Balfour. He says (p. 536) : " The canal 

 organs also sink below the surface, but they carry with them the surrounding tissues, and 

 by a process of infolding become enclosed in short canals, each containing a single organ. 

 These shurt canals then become continuous, a single surface opening being left between 

 every two consecutive oi'gans along each line. These simple openings, or primary ]>ores, 

 may be retained in the adult, but most of them undergo a repeated dichotoinous division, 

 thus giving rise to groups of surface jiores and to corres^'oudiug dendritic systems of 



* Wright, however, was wiitiug without access to Merkel's work. 



