30 MR. T. H. HUXLEY ON THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA. 
fii-st, from the want of a clear and definite conception of the fundamental varieties 
of molluscous structure, and of the nature of the changes in the relations of parts 
which constitute those varieties ; and secondly, from the want of a due regard to the 
facts presented by the development of the different families, which must stand in the 
relation of cause to the varieties of form. 
Now in order to the former end (the obtaining of a definite conception of the 
varieties of molluscous form), I propose to set forth the structure of certain Hetero- 
poda and Pteropoda ; pelagic animals so transparent, that a perfect knowledge of 
the arrangement of their parts may be arrived at by simple inspection, without so 
much as interrupting a beat of their heart. 
Afterwards, I shall inquire how far the known laws of development account 
for these forms, and thence of what archetypal form they may be supposed to be 
modifications. 
PART I. 
I. Anatomy of Firoldides (Plate II. fig. 1). 
The species of Flrola which I examined appears to be identical with the Firoldides 
Desmarestii of MM. Eydoux and Souleyet*. 
The animal may be described as a transparent cylinder about an inch long, and so 
generally colourless as to be hardly distinguishable in the water, except by the 
incessant flapping of its flattened ventral appendage 
The only parts which present any colour are the buccal mass, which is brownish ; 
the eyes, almost black, and the mass of the liver, which is brownish green ; further, 
the anus has a pinkish tint. 
Attached bva narrow root or pedicle to the ventral surface of the cylindrical body 
is the broad cheese-cutter-shaped foot, or as I propose to call it, propodium (pp). Its 
posterior edge is quite sharp, and carries no suckei-like expansion. The anteiioi 
fifth, or thereabouts, of the animal is thinner than the rest of the body, and it narrows 
again towards its extremity, which is truncated, and forms a circular lip round the 
aperture of the mouth. Just behind the narrowed fifth, and towards the dorsal 
surface, we observe the eyes, and immediately below, and as if proceeding from 
them, are the tentacles, which are short and conical. 
The posterior extremity also is abruptly truncated ; its uppermost angle slightly 
projects, and viewed from above appears like a subspiral, richly ciliated band {d). 
Some little distance from this is the aperture of the anus (a). 
From the two inferior angles, two tubular processes pass in the male (fig. 1). The 
right process ends in a globular body and is the penis {p), while the left (which I 
shall call the metapodium) is long and somewhat pointed {mt). 
In the female there is only one process (the metapodium) (fig. 2, 3mt) answering 
* Figured by them in their beautiful plates illustrative of the Zoology of the Voyage of the Bonite. 
