GASTEROPODA. 199 



wanting in others ; in liroloides they are only indicated hy a ciliated sub- 

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

 (Gegenbaur.) 



The nucleobrauches are dioecious ; some individuals (of Jirola) 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. (Gegeubaur.) 



The nervous system is remarkable for the wide separation of the centres. 

 The buccal ganglia are situated considerably in front of the cephalic, arid 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 pos- 

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

 shaped, and very perfectly organized ; the auditory vesicles are placed behind, 

 and connected with the cephalic ganglia, they each contain a round otolite, 

 which sometimes seems to oscillate. (Huxley.} 



FAMILY I. 



Animal elongated, cylindrical, translucent, furnished with a ventral fin, 

 and a tail fin used in swimming; gills 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.f 



FIROLA, Peron and Lesueur. 



Type, F. Coronata, Forsk. Medit. Syn. Pterotrachsea, Forsk. 



Animal fusiform, elongated, with a long, slender, proboscidiform 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, pro- 

 tected by a rudimentary eyelid ; lingual ribbon oblong. The female firola 

 have a long moniliform oviduct. Anops Peronii, D'Orb. described and 

 figured as having no head (!) was probably a mutilated Firola. " Such speci- 

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



Distr. 8 sp. Atlantic, Medit. Pacific. 



Sub-genus, Firoloides, Lesueur. ( Cerophora, D'Orb.) F. Desmarestii, 

 Les. Body cylindrical ; head tapering, furnished with two slender tentacles ; 

 nucleus at the posterior extremity of the body, with or without small bran- 

 chial filaments; egg-tube regularly annulated; tail fin small and slender, 

 ventral fin without a sucker. Distr. 6 sp. Atlantic. 



* We can only call to mind one other example of a segmented organ in the mol~ 

 lusca ; viz. the penniform styles of Teredo bipalmulata.. 



t The genus Sagitta, Q. and G. sometimes referred to this family, is an articulate 

 animal. (HuxleyO 



