THE LATERAL CANAL SYSTEM. 300 
Canal System. From the simple “nerve papilla” 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 from alcoholic specimens, an organ of 
considerable size in which there is a central yellowish opaque 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 mucus 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., XXVIIL, and XXXI.). Plates 
LXXI to LXXXIV. indicate somewhat closely 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 entirely disappeared. The sketches most often show 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 sys- 
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, than 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 difficulty. 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- 
pterois with their greatly enlarged tactile fin rays, have rather small disks as 
compared with those of Bassozetus, Eretmichthys and Lamprogrammus, 
