THE LATERAL CANAL SYSTEM. 355 



Canal System. From the simple " nerve i^apilla " there are all degrees of 

 complexity leading up to the intricate structures possessed by Leucicorus or 

 by Mixonus, Plates XXXVIII. fig. 7, and XXXIX. fig. 2. On deep sea 

 species each papilla has enlarged and instead of being a minute rounded 

 fleshy tubercle has become, as told I'rom alcoholic specimens, an organ of 

 considerable size in which there is a central yellowish opaq-ne rounded body, 

 corresponding to the papilla, surrounded by a whitish translucent fusiform 

 portion situated transversely in the canals of the system and enveloped in a 

 rounded or longitudinally elliptical mass or corona resembling thickened mu- 

 cus; the structures together forming a prominent convex body, which for 

 present convenience may be called a disk, partially obstructing the canals, 

 which latter are filled with a thin nuicus in contact with the sea water through 

 pores or openings, commonly seen at the lower edge of the canal, one of 

 them near each of the glandular disks. In many cases the minute pores of 

 the shoal water species have given place to apertures of comparatively large 

 dimensions (see figures on Plates XXVII., XXVIII., and XXXI.). Plates 

 LXXI. to LXXXIV. indicate somewhat closeh' the various degrees of en- 

 largement in the disks, since the different sketches maintain the proportions 

 ■with regard to the size of the head as nearly as possible. On fresh speci- 

 mens no doubt an enveloping mucus is present ; from the material in hand 

 it has entirel}^ disappeared. The sketches most often sliow only the outlines 

 of the fusiform and the included central body (centrum), to which is added 

 in many cases the slender threadlike connection between the disks. 



Comparing these organs great variations in sizes appear in the different 

 species and genera. In a general view the size of the disk is greater in 

 species of the greater depth where more dependence is placed on the s^-s- 

 tem ; most often also, like the eye, the disk is of a greater comparative size 

 in the young, which in all probability place more reliance upon it, tlian in 

 the adult, in which strength swiftness or armature are more efficient. On 

 young of some species the disks are very distinct while on the, adult of the 

 same species they can be detected only with considerable difficuIt3^ Though 

 it is true that the system attains its greater development in the greatest 

 depths, it is not at all accurate to say that it increases in prominence and 

 complexity in all the species. For those possessing extensive developments 

 of other sensory organs, as species of Dicrolene, Pteroidonus, and Bathy- 

 ptero'is with their greatly enlarged tactile fin rays, have rather small disks as 

 compared with those of Bassozetus, Eretmichthys and Lamprogrammus, 



