rTEROTRACIIEWiE. 03 



tentacles subulate ; eyes large, and sessile on the sides of 

 the head. Body large, sub-cylindrical, hyaline, gelatinous, 

 ending behind in a long tail; abdomen rudimentary, form- 

 ing a sessile, exposed nucleus as in Pterotrachea, or a 

 pedunculated nucleus, covered with a thin pellucid shell, as 

 in Garinaria ; gills tufted, situated in front of the nucleus. 

 Foot compressed and fin-like, with a small flat disk on the 

 hinder edge. 



Operculum none. 



Shell symmetrical, hyaline, sub-spiral. 



Pelagian. 



The animals of this family are all pelagian, and are 

 common in tropical seas, swimming in a reversed position 

 with considerable velocity. Like the Atalants and the 

 Pteropods, they usually come to the surface towards the 

 close of day. They are predacious, feeding on the small 

 Acalejplue that abound in the solitudes of the ocean. 

 Owing to the peculiarities of their organization they are 

 unable to crawl, but they can attach themselves to floating 

 bodies by means of the sucker on their ventral fin. Gari- 

 naria and Cardiqpoda are the only genera at present 

 known which are provided with shells. It frequently hap- 

 pens that individuals are torn and mutilated by fishes, in 

 which case the specimens so mutilated have formed the 

 types of spurious genera. Of such a nature are the Mono- 

 phorus and Timoriensis of Quoy and Gaimard, and the 

 An ops of D'Orbigny. The gills are well developed in 

 Carinaria, but in Firoloidea they are obsolete or rudi- 

 mentary; the eyes are hour-glass-shaped and well -organized, 

 and are even furnished in some cases with a rudimentary eye- 

 lid, and there are distinct auditory vesicles, each containing 

 a round otolite. The oviduct, in some genera, is extruded in 



VOL. II. o 



