342 MANUAL OF THE M0LLI7SCA. 



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 oesophagus. The branchial ganglia are at the posterior 

 extremity of the body, as in the bivalves. The eyes are hour- 

 glass shaped, and very perfectly organised ; the auditory 

 vesicles are placed behind, and connected with the cephalic 

 ganglia, thej each contain a round otolite, which sometimes 

 seems to oscillate. (Huxley.) 



Family I. — Firolid^e. 



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, or covered by a small hyaline 

 shell. Mouth with a circular lip ; lingual membrane with few 

 rows of teeth; central teeth transversely elongated, with 3 

 recurved cusps ; laterals 3 on each side, the first a transverse 

 plate with a hooked apex, 2 and 3 sickle-shaped. * 



FiEOLA, Peron and Lesueur. 



Type, F. Coronata, Forsk. Mediterranean. 



Synonym, Pterotrachsea, Forsk. 



Animal fusiform, elongated, with a long, slender, proboscidi- 

 form head ; fin narrowed at the base, furnished with a small 

 sucker ; tail elongated, keeled, sometimes pinnate ; nucleus 

 prominent; branchial processes numerous, conical, slender; 

 tentacles 4, short and conical ; eyes black and distinct, protected 

 by a rudimentary eyelid ; lingual ribbon oblong. The female 

 firolce have a long moniliform oviduct. Anops Peronii, 

 D'Orbigny, described and figured as having no head (I), was 

 probably a mutilated Firola. " Such specimens are very 

 common, and seem just as lively as the rest." (Huxley.) 



Distribution, 14 species. Atlantic, Mediterranean, Pacific. 



Sub-genus. Firoloides, JjGSueur. {Cerophora,'D'OThigiij). F. 

 Dcsmarestii, Les. Body cylindrical ; head tapering, furnished 

 with two slender tentacles ; nucleus at the posterior extremity 

 of the body, with or without small branchial filaments ; egg- 

 tube regularly annulated ; tail-fin small and slender, ventral 

 fin without a sucker. Distribution, 6 species. Atlantic ; Medi- 

 terranean. 



* Th.c gonus Sagitta, Q. and G., sometimes referred to this family, is an sirticrlato 

 ouimuL (Huxley.) 



