GASTEROPODA. 841 
foot (pro-podiwm), which is very distinct in Natica (p. 235), and 
in Harpa and Oliva; but is only marked by a groove in 
Paludina and Dolium (Fig. 87). The terminal fin (or tail of 
Carinaria), which carries the operculum of Atlanta, is the 
equivalent of the operculigerous lobe (meta-podiwm) of the ordi- 
nary gasteropods, such as Strombus (Fig. 76). 
The abdomen, or visceral mass, is small, whilst the anterior 
part of the body (or cephalo-therax, M. Edwards) is enormously 
developed. The proboscis is large and cylindrical, and the 
tongue armed with recurved spines. The alimentary canal of 
Firola is bent up at a right angle posteriorly on the dorsal side ; 
in Atlanta it is recurved, and ends in the branchial chamber. 
The heart is proso-branchiate, although in Firola the auricle is 
rather aboye than in front of the ventricle, owing to the small 
amount of the dorsal flexure. 
The nucleobranchs, and especially those without shells, 
‘‘ afford the most complete ocular demonstration of the truth 
of Milne Edwards’s views with regard to the nature of the cir- 
culation in the mollusca. ‘Their transparency allows the blood- 
corpuscles to be seen floating in the general cavity of the body 
—between the yiscera and the outer integument—and drifting 
backwards to the heart; haying reached the wall of the auricle 
they make their way through its meshes as they best can, 
sometimes getting entangled therein, if the force of the heart 
has become feeble. From the auricle they may be followed 
to the ventricle, and thence to the aorta and pedal artery, 
through whose open ends they pour into the tissues of the head 
and fin.” (Huxley.) 
Such delicate and transparent creatures would hardly seem to 
need any special breathing-organ, and, in fact, it is present or 
- absent in species of the same genus, and even in specimens of 
the same species. Carinaria has fully-formed branchize; in 
Atlanta they are sometimes distinct, and wanting in others; in 
Firoloides they are only indicated by a ciliated sub-spiral band. 
The laryee are furnished with a shell, and with ciliated vela. 
(Gegenbaur.) 
The nucleobranchs are diccious ; some individuals (of Firola) 
haye a leaf-like appendage, others a long slender egg-tube 
depending from the oviduct, and regularly annulated.* The 
laryee are furnished with a shell and with ciliated vela. (Gegen- 
baur.) 
The neryous system is remarkable for the wide separation of 
* We can only call tomind one other example of a segmented organ in the mellusca, 
viz., the penniform styles of Zeredo bipalmulata. 
