THE GASTROPODA 167 
(Figs. 95, pe, and 96, g.pe) commissure. The sub-class includes the 
two orders, Opisthobranchia and Pulmonata. 
ORDER 1. Opisthobranchia, Milne-Edwards. 
Marine Euthyneura with aquatic respiration ; the ventricle of 
the heart is generally anterior, and the pallial cavity, when present, 
is widely open. ‘There is a marked tendency to a reduction of the 
shell, which may become internal or disappear. In the naked 
forms spicules are sometimes developed (Pleurobranchidae, Dorido- 
morpha, Hedylidae, Fig. 168, sp.). The order comprises two sub- 
orders, Tectibranchia and Nudibranchia. 
Fia. 146. 
Central nervous system of Limnaea stagnalis, right-side view. bu, buccal mass; g.a, ab- 
dominal ganglion ; g.vu, buccal or stomato-gastric ganglion ; g.ce, cerebral ganglion ; g.p, pedal 
ganglion ; g.pl, pleural ganglion ; g.s.i, supra-intestinal ganglion ; lo.d, dorsal lobe of cerebral 
ganglion ; Jo.l, lateral lobe of cerebral ganglion ; n.co, columellar nerve ; n./a, labial nerve ; 7.0. 
optic nerve; ».pa, right pallial nerve; n.par, parietal nerves; n.pe, penial nerve; 1.te, 
tentacular nerve ; oe, oesophagus ; of, otocyst and nerve. 
Sus-OrperR 1. TECTIBRANCHIA, Cuvier. 
Opisthobranchs provided in the adult state with a mantle and a shell, 
with the exceptions Runcina, Plewrobranchaea, the Cymbuliidae, and some 
Aplysiomorpha. There is a ctenidium, except in some “ Thecosomata” 
and ‘ Gymnosomata,” and an osphradium. The sub-order includes three 
tribes, the Bullomorpha, the Aplysiomorpha, and the Pleurobranchomorpha. 
TRIBE 1. BULLOMORPHA. 
In these Tectibranchs the shell is usually well developed (it is want- 
ing in Runcina and the Cymbuliidae), and may be external or internal. 
There is no operculum except in the Actaeonidae and Limacinidae (Fig. 
49, op). The pallial cavity is always well developed, and contains the 
ctenidium, in part at least: this ctenidium, except in the Lophocercidae, 
is of the “folded” type. With the exception of the Aplustridae, Lopho- 
cercidae, and Thecosomata, the head is devoid of apparent tentacles, and 
its dorsal surface forms a digging disc or shield usually separate from the 
neck, and with more or less scolloped margins. The edges of the foot 
(parapodia) are continuous with the ventral face of that organ (Fig. 147, /), 
and are often transformed into highly-developed fins (Fig. 151, V1). 
