258 DR. A. RATTRAY ON THE ANATOMY, PHYSIOLOGY, 
anterior part of this triangular muscular pateh embraces one-third or half of the oral 
orifice (Plate XLIIT. fig. 10,9; Plate XLIV. fig. 11, g), and probably serves to elongate, 
retract, and curve the proboscis, perhaps to aid in opening the mouth and assist in de- 
-glutition, the bolus of food, first seized by the tongue, being forced into the cesophagus 
by successive contractions of these fibres from before backwards. The arrangement and 
great development of the muscular coat in the anterior part of the Firolidæ accounts for 
the mobility of the proboscis and eyes. 
The ventral fin is flat, and principally composed of muscular fibres enveloped by a 
thin layer of the transparent outer tunic; and to their arrangement is due its peculiar 
fan- or fishtail-like action. There are usually three layers, an inner and two outer, —the 
former consisting of strong fibrillæ running upwards and backwards in parallel lines 
from the attached to the free edge; while the latter, which are finer, run upward and 
forward, aggregated into bundles in the narrow neck of the fin, but widening out towards 
its free edge (Plate X LIII. fig. 1,5). The fins of Carinaroides (Plate XLIV. figs. 14, 16) 
have a similar structure (figs. 3,4). In the latter, however, the external layers are pro- 
longed at intervals, and in small bundles, towards the free edge of the enveloping tunic, 
subdividing several times, and finally joining corresponding fibrille from the muscular 
layer of the opposite side. | 
The arrangement of the muscular fibres of the tail, absent in Firoloides, varies in dif- 
ferent species of the Firolidæ ; but in all cases it is apparently connected with its function 
as a steering- and swimming-apparatus. It sometimes receives a coat from the external 
tunie, but often does not, as in Firola (Plate XLIII. fig. 4. In the latter, besides 
possessing the longitudinal and transverse muscular coat common to the whole body, 
it has an additional set that form small bands, of which one, elongated and fusiform 
(fig. 4, 2), runs towards the tip a little distance below the muscular tunic, and joins 
a larger and better-marked fusiform strip (fig. 4, y), which lies deeper, and then runs 
onward nearly to the tip of the tail. These bands are connected along their posterior 
half by cross fibres. A third, more slender (2), runs from the tip forward, divides and 
diverges so as to embrace the branchial nucleus. Besides aiding the fin-like motions 
of the tail, this may also serve as a basis of attachment for the intestinal canal, to assist 
its antero-posterior vermiform movement during the processes of deglutition, diges- 
tion, and defecation. Progression is effected principally by the ventral fin, which moves 
actively ; while the tail is chiefly employed as a steering-apparatus. Thus the arrange- 
ment of the muscular element in both varies much. Each variety appears to have its 
own, probably characteristic, by which alone it might doubtless be distinguished ; but 
though useful as an additional diagnostie mark, being a microscopic one it is disadvan- 
tageous, and therefore not so practically useful or likely to be so frequently followed as 
others, e. g. the presence or absence of a shell, branchiæ, a caudal appendage, the position 
of the visceral nucleus, &c. 
The sucker (Plate XLIII. fig. 1,c), oval from above downward, and situated at the upper 
posterior free edge of the ventral fin, is small, rudimentary, and probably seldom, if ever, 
used, being principally developed in the outer tunie, and having little apparent con- 
nexion with the muscular tissue below. It is present, but only in a rudimentary form, | 
