AND DISTRIBUTION OF THE FIROLIDA. 259 
in all the hereafter enumerated varieties of Pacific Firolidz, including specimens of 
Firoloides, Firola, Carinaroides, and Carinaria; and the foot is probably exclusively a fin 
both in form and function. 
These muscular envelopes resemble those of Helix, Limax, and other typical Gastero- 
pods, and enable the animal to elongate, contract, turn, progress, and otherwise alter 
its shape at will. The proboscis seems specially endowed with the power of motion. 
Under the microscope the longitudinal and transverse layers have more the appearance of 
fibrous than muscular tissue (Plate XLIII. fig. 11), and consist of parallel strips, some- 
times slightly interlaced ; but a higher power would doubtless demonstrate the existence 
of the transverse striæ characteristic of voluntary muscle, as it readily does in a lower form 
of life, the Salpa, where the muscular element does not form a continuous coat, as here, 
but exists merely in occasional bands; for animals like the Firolidæ, whose nervous 
system and organs of special sense are so highly developed, must possess the power of 
wandering about at will, and performing voluntary motions; nor can we suppose that 
all their evolutions are purely involuntary, instinctive, or both. 
A system of aquiferous canals has been said to exist between the external and in- 
ternal tunies, in connexion with the venous system; others say, independent of it. 
Their presence is still an open question, and one difficult to verify either way. Certainly 
different specimens even of the same animal show a variable degree of relative distention 
or collapse, which may be readily accounted for by the presence of such an apparatus 
and the admission or expulsion of water at will, more especially as this mutable size 
is principally referable to the outer tunic, in which these canals are believed to lie. One 
use is probably that of a somewhat similar system in the .Echini and Asteride, and still 
more nearly that of the ramifying aqueous canals of Holothuria, viz. to assist aération 
of the blood, especially in those varieties in which a branchial apparatus is wanting, ru- 
dimentary, or insufficient for the requirements of the animal, by bringing it and the 
aérating medium more closely into contact, and, in fact, separated only by the thin mus- 
eular coat. But they, perhaps, serve another purpose; for, admitting the probable 
presence of a system of aquiferous lacunæ or canals either in the external envelope or 
between it and the muscular coat, and remembering the trifling connexion that exists 
between these two, the former being nowhere bound down to the latter, except at or 
near the oral orifice, rudimentary sucker, and vicinity of the branchial nucleus and tail, 
it is easy to conceive how these may be emptied or filled at need, and how, by contract- 
ing the general coat, the canals will necessarily have their calibre increased and filled ; 
whereas, by the converse process of relaxation of the muscular coat and consequent en- 
largement of the true body of the animal, they will be pressed on, diminished in size, 
and emptied of their contents. The function of this system, therefore, doubtless cor- 
responds also to that of the swimming-bladder of the fish and, still more, the subcuta- 
neous air-cells so prominently developed in some birds (e. y. the Gannet, Pelican, &e.), 
and, beside aiding the aöration of the blood, further serves to enable the animal to alter 
its specific gravity at will. By infiltration of the skin of the one with air, and the 
hyaline coat of the other with water, the weight of the bird or Hirota is reduced and 
equalized with that of the gaseous or aqueous medium in which it floats ; and thus the 
