348 PIROLIDiE. 



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 Mollusca. Their transparency allows the blood-corpuscles 

 to be seen floating in the general cavity of the bod}^ — 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 arter3% 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 gen lis, and even in specimens of 

 the same species. Carinaria has fully-formed branchiae ; in 

 Atlanta they are sometimes distinct, and wanting in others ; in 

 Firoloides they are only indicated by a ciliated subspiral band. 

 The larvae are furnished with a shell, and with ciliated vela. — 

 Gegenbauer. 



The nucleobranchs are dioecious ; some individuals (of Firola) 

 have a leaf-like appendage, others a long, slender egg-tube 

 depending from the oviduct, and regularly annulated. The larvae 

 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 ossophagus. The branchial ganglia are at the posterior 

 extremity of the bod}^ as in the bivalves. The ej^es are hour- 

 glass shaped, and very perfectly organized ; the auditory vescicles 

 are placed behind, and connected with the cephalic ganglia; 

 they each contain a round otolite, which sometimes seems to 

 oscillate. — Huxley. 



Family FIROLIDJE. 



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. — Huxley. 



