AND DISTRIBUTION OF THE FIROLIDA. | 267 
fin, and buccal mass, the other viscera and tissues being sufficiently nourished and 
oxidized by contact with the less highly vitalized fluid of the afferent current. 
Respiratory Apparatus. 
This consists, in this form of Firola, of an elongated cylindrical tube (Plate XLIII. 
fig. 4, F L) attached to the under surface of the ventricular sac, its long axis directed 
antero-posteriorly ; its anterior end truncated, and its posterior extremity opening exter- 
nally near the anal orifice. Its centre is occupied by a tube which runs nearly its 
entire length, and is about one-third the diameter of the larger cylinder. The exterior 
of the central tube and interior of the outer one are thickly covered with very active 
cilia, by which a constant renewal of the water-supply is effected through the open pos- 
terior end, the current having the double purpose of changing the oxygenating me- 
dium, and of carrying away feecal accumulations from the adjacent anus. The whole is 
attached to the under surface of the auricular sinus; and the blood is possibly aérated, 
partly, at least, through the membrane which separates them, and partly in the cavity 
of the respiratory organ itself. Simple juxtaposition cannot be the only connexion be- 
tween the heart and breathing-organ ; and some communication must exist between the 
two to permit the passage of blood to be oxygenated in the interior. The entire subject 
still requires study. Where no branchial apparatus or localized and special breathing- 
organ exists, as in Firoloides, (Plate XLIII. fig. 1), or is found only in a very rudimentary 
form, the vitalizing process is doubtless carried on during its passage through the venous 
sinuses or periintestinal cavity, especially those next the surface. Although the breath- 
ing-apparatus varies much in different families and even genera of the Heteropoda, each 
species, doubtless, possesses its own peculiarity ; nor is its presence or absence a matter 
of such uncertainty as some suppose. 
Digestive System. 
In Firola and Firoloides this consists of :—first, the parts which form the buccal nucleus, 
viz. the oral orifice, buccal cavity, tongue, upper part of esophagus, &e.; second, the 
stomach and intestinal canal; and, third, the secretory organs which aid digestion, viz. 
the salivary glands and liver. 
The oral orifice is terminal, circular, non-labiate, comparatively large, and capable of 
considerable dilatation and contraction from the presence of circular fibres, which form 
a kind of oral muscle. Behind it lies the ** buccal nucleus” (Plate XLIII. figs. 1, 2, 3, £), 
so termed, except during the protrusion of the tongue, when the whole is pushed for- 
ward (Plate XLIV. fig. 12), partly by the cesophagus, but chiefly by the muscular coat. 
The buccal cavity is round or oval antero-posteriorly, and principally occupied by the 
tongue. The latter, or glottideum, is so armed with teeth as to form a rasping-organ, 
the root of which is large, cartilaginous, and chiefly composed of oval transparent cells 
(Plate XLIII. fig. 2, y), on which rests the lingual ribbon (Plate XLII. figs. 2, 3; 
Plate XLIV. fig. 11), which doubles beneath the lingual mass, but unfolds while feeding, 
and protrudes as in Plate XLIV. fig. 12, with its point more or less curved, and the teeth 
on the tip and dorsum erect and prominent. The lingualribbon (Plate XLIV. fig. 11, y) is 
