GASTEROPODA PECTiNIBRANCHIATA. 357 
The Firola, Peron, — 
Has the body, 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 
j 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 has 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 terminated with a tail, M. Lesueur 
distinguishes as Fh'oloides. 
I 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 Firolae deprived of their foot and nucleus of 
I viscera ; and the Monophores of the same naturalists, which have nearly the form of Carinaria, but are 
I 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 
I much compressed, has in front a snout surmounted with two long tentacula without eyes ; behind, a 
j 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 comjirehends 
almost all the univalve spiral shells, and several which are simply conical. The branchias, 
j composed of numerous leaflets or fringes, ranged parallelly like the teeth of a comb, are affixed 
j in one, two, or three lines (according to the genera) to the floor of the pulmonary cavity, wdiich 
j occupies the last whorl of the shell, and which communicates outw^ards by a wide gape betw^een 
I 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 
I as that of the order ; and they are the only ones which respire the atmosphere, water being 
I the medium of respiration to all the rest. 
All the Pectinibranchiata have tw^o tentacula and two eyes, raised sometimes on pedicles ; a 
I mouth in the form of a proboscis, more or less lengthened ; and separate sexes. The penis 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 Paludina alone has 
' the organ concealed, and it comes out through a hole pierced in the right tentaculum. The 
I rectum and the oviduct of the female also creep along the riglit side of the branchial cavity ; 
; and there is between them and the branchiae a peculiar organ, composed of cells filled with a 
j very viscous fluid, the use of wdiich is to form a common envelope for the inclosure of the eggs, 
and which the animal deposits with them. The form of that envelope is often very complicated 
and very remarkable. 
The tongue is armed wdth little hooks [or curved spinules], and w^ears down the hardest 
j bodies by slow and oft-repeated frictions. 
I The grand difference betw'een 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 wdiich 
I * In M. tie Bla'mville's system, it f ornis the subclass Paraeephalophota dioica. 
