GASTEROPODA PECTINIBRANCHIATA. 357 



The Firola, Peron, — 

 Has the l)ocly, the tail, the foot, the branchiae, and the nucleus of the viscera, nearly the same as the 

 Carinaria, but no shell has been observed. Their snout is prolonged into a recurved proboscis, and 

 their eyes are not fronted with tentacula. There is often seen hanging at the end of their tail, a long 

 jointed thread, which Forskal considered to be a Tape-worm, and the nature of which is not yet cer- 

 tainly determined. 



One species (Pterotrachea coronata, Forsk.) is very common in the Mediterranean; and M. Lesueur lias de- 

 scribed several others from the same sea as different, but they require new and comparative examinations. Such 

 as have the body abruptly truncate behind the visceral nucleus, instead of being termina'ed with a tail, M. Lesueur 

 distingTiishes as Firoloides. 



To these genera, now well known, I suppose we shall, on a better acquaintance with them, have 

 to add the Timoriennes, Quoy & Gaym., which appear to be Firola; deprived of their foot and nucleus of 

 viscera; and the Monophores of the same naturalists, which have nearly the form of Carinaria, but are 

 also footless and shelless, nor have any visceral nucleus. 



It is not so certain that we should place here the Phylliroes of Peron. The body, transparent and 

 much compressed, has in front a snout surmounted with two long tentacula without eyes ; behind, a 

 truncate tail ; and we can see through the integuments its heart, its nervous system, its stomach, and 

 the genital organs of both sexes. The anus, and the orifices of the genital organs, are also on the right 

 side, and a penis of considerable length is sometimes even protruded ; but I cannot perceive any other 

 respiratory organ than its thin and vascular skin. 



THE SIXTH ORDER OF THE GASTEROPODES. 



THE PECTINIBRANCHIATA.* 



This order is, beyond comparison, the most numerous of the class, since it com])rehend3 

 ahnost all the univalve spiral shells, and several which are simply conical. The branchiae, 

 composed of numerous leaflets or fringes, ranged parallelly like the teeth of a comb, are affixed 

 in one, two, or three lines (according to the genera) to the floor of the pulmonary cavity, which 

 occupies the last whorl of the shell, and which communicates outw ards by a wide gape between 

 the margin of the cloak and the body. Two genera only — Cyclostoma and Helicina — have, 

 instead of branchise, a vascular network clothing the ceiling of a cavity in all respects the same 

 as that of the order ; and they are the only ones which respire the atmosphere, water being 

 the medium of respiration to all the rest. 



All the Pectinibranchiata have two tentacula and two eyes, raised sometimes on pedicles ; a 

 mouth in the form of a proboscis, more or less lengthened ; and separate sexes. The jjcnis of 

 the male, attached to the right side of the neck, cannot, in general, be drawn within the body, 

 but is reflected into the branchial cavity ; it is sometimes very large. The Pakulina alone has 

 the organ concealed, and it comes out through a hole pierced in the right tentaculura. The 

 rectum and the oviduct of the female also creep along the right side of the branchial cavity ; 

 and there is between them and the branchiae a peculiar organ, composed of cells filled with a 

 very viscous fluid, the use of which is to form a common envelope for the inclosnre of the eggs, 

 and which the animal deposits with them. The form of that envelope is often very comi)licated 

 and very remarkable. 



The tongue is armed with little hooks [or curved si)inuU's], and wears down the hardest 

 bodies by slow and oft-repeated frictions. 



The grand diff'erence between these animals lies in the presence or absence of the canal 

 formed by the prolongation of the margin of the branchial cavity on the left side, and which 



• In M.ilc IJlaiiiville's system, it f orms the subclass Paracf/ihulii/jhtm diuka. 



