MR. T. H. HUXLEY ON THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA. 33 
Respiratorij System. — So delicate a creature would hardly seem to need any special 
svstem of this hind, and I found no trace of such organs in any, even the freshest 
and most uninjured specimens. 
In the nearly allied species Firola Keraudrenli, however, the gills appear as a row 
of conical processes, extending along the posterior edge of the body behind the anus ; 
and in other species such processes develope accessory folds, until in Carinaria we 
find fully-formed branchiae (see Eydoux and Souleyet). The ciliated subspiral 
band {d}, which will be found to have its homologue in Atlanta, is the only structure 
which appears to be capable of assisting the respiratory function ; but its small size 
must render it of very little importance. 
Mantle. — There is no distinct mantle in Firoldides Desmarestii. 
Carinaroides (Eydoux and Souleyet) evidently forms the transition from the 
Heteropods without a mantle to those with one. It is in this genus placed at the 
lower posterior angle of the body, and carries a minute shell; the branchiae are 
developed between it and the anus. 
In Carinaria there is a proportionably small mantle and shell, but it occupies a 
position more resembling that of ordinary Mollusks ; and in Atlanta, as will be seen, 
the relative proportions of the mantle and body are pretty nearly those found in 
ordinary Gasteropods. 
Contractile Sac or Urinary Organ (c, figs. 2, 3).— Between the rectum and the heart, 
and therefore bathed by the returning venous blood, there lies an elongated, 
flattened, delicate and transparent sac, whose walls are usually very much wrinkled 
and sacculated. 
This sac opens by a rounded aperture in its upper part upon the right side of the 
animal, and is of course continually filled by the surrounding water. As the sac is 
incessantly contracting, however, this water must be continually renewed, and hence 
the organ, simple as its structure appears and small as its size may be, is probably 
a very efficient depurating agent*. 
Considerations to be stated hereafter, lead me to the belief that it is in fact the 
urinary organ, at once kidney and urinary bladder. 
Reproductive System. — Firoldides is dioecious. The male may be distinguished at 
once from the female, by the peculiar penis {p) attached to the right posterior 
inferior angle of the body (fig. 4). It consists of two portions ; the larger is cylin- 
drical, but enlarged at its extremity into a globular head, from one side of which a 
small pointed process projects. The globular body contains many large cells as a 
sort of lining, and within these there is a cavity which communicates with the exte- 
rior through the pointed process. A vast number of small oval fatty-looking particles 
may be made by pressure to pass out from the cavity. 
* In the ‘Explication des Planches,’ Eydoux and Souleyet call a similar organ in Firola “ organe de la 
depuration urinaire ?.” 
