37 
in thai marsupial sack, and brought forth by the 
paternal parent. The form of the hippOcampus, and 
the flexible power of the vertebral column are special 
adaptations in the fish to this economy. 
No oné looking at the hard dry specimens of the 
hippocampus in the naturalists’ cabinet would sup- 
pose that fish to be of a structure peculiarly tegumen- 
tary and yielding. Fishes in the integument enve- 
loping them, secrete a solid material that takes the 
form of extravascular lamine embedded in the living 
and vascular cutis ; that is, they are covered with | 
seales. ‘This is the exoskeleton ; the osseous frame 
work proper to the vertebrate body being the 
endoskeleton. 
' The spines of all fishes are indabitable derivations 
of the cuticular exoskeleton. Whether the spines 
be fixed by moveable articulations to the body, and 
raised or depressed by muscles inserted into their 
bones as in the mechanism of. the vertical fins, or 
- whether, they are mere points, enormously developed 
and serrated for wounding desperately, they indicate 
peculiar structures only of the external covering to 
the vascular and living skin to which their under 
surfaces are adherent. ‘The most usual form of 
the cuticular covering of fishes,” remarks Professor 
Rymer Jones, with admirable descriptive brevity, — 
‘“‘ ig that of imbricated scales, with which the whole 
exterior of the body is compactly encased, as in a 
suit of armour. Such an investment is admirably 
adapted to their habits and economy. The dense 
and corneous texture of the scales, impermeable to - 
water, defends their soft bodies from maceration 
D 
