348 FIROLIDA. 
Firola is bent up at a right-angle posteriorly on the dorsal side ; 
in Atlanta it is recurved, and ends in the branchial chamber. 
The heart is prosobranchiate, although in Firola the auricle is 
rather above than in front of the ventricle, owing to the small 
amount of the dorsal flexure. 
The nucleobranchs, and especially those without shells, ‘ afford 
the most complete ocular demonstration of the truth of Milne 
Edwards’ views with regard to the nature of the circulation in 
the Mollusea. Their transparency allows the blood-corpuscles 
to be seen floating in the general cavity of the body—between 
the viscera and the outer integument—and drifting backwards 
to the heart; having reached the wall of the auricle they make 
their way through its meshes as they best can, sometimes getting 
entangled therein, if the force of the heart has become feeble. 
From the auricle they may be followed to the ventricle, and 
thence to the aorta and pedal artery, through whose open ends 
they pour into the tissues of the head and fin.” —HUXLEY. 
Such delicate and transparent creatures would hardly seem to 
need any special breathing organ, and, in fact, it is present or 
absent in species of the same genus, and even in spetnmens: of 
the same species. Carinaria has fully-formed branchie; °i 
Atlanta they are sometimes distinct, and wanting in others: in 
Firoloides they are only indicated by a ciliated subspiral band. 
The larve are furnished with a shell, and with ciliated vela.— 
GEGENBAUER. 
The nucleobranchs are dicecious ; some individuals (of Firola) 
have a leaf-like appendage, others a long, slender egg-tube 
depending from the oviduct, and regularly annulated. The larvee 
are furnished with a shell and with ciliated vela.—GEGENBAUER. 
The nervous system is remarkable for the wide separation of 
the centres. The buccal ganglia are situated considerably in 
front of the cephalic, and the pedal ganglia are far behind, so 
that the commissures which unite them are nearly parallel with 
the csophagus. The branchial ganglia are at the posterior 
extremity of the body, as in the bivalves. The eyes are hour- 
glass shaped, and very perfectly orgunized; the auditory vescicles 
are placed behind, and connected with the cephalic ganglia; 
they each contain a round otolite, which sometimes seems to 
oscillate. — HUXLEY. 

Faminry FIROLID A. 
Animal elongated, cylindrical, translucent, furnished with a 
ventral fin, and a tail-fin used in swimming ; gill exposed on the 
posterior part of the back. No shell. 
The genus Sagitta, Q. and G., sometimes referred to this 
family, is an articulate animal.—HuxLery. 
