Page:EB1911 - Volume 11.djvu/540

swimming lobes. Visceral commissure much shortened, except in Aplysia. Genital duct monaulic; hermaphrodite duct connected with penis by a ciliated groove. Animals either swim or crawl.

Fam. 1.—Aplysiidae. Shell partly or wholly internal, or absent; foot long, with well-developed ventral surface. Aplysia. Dolabella. Dolabrifer. Aplysiella. Phyllaplysia. Notarchus.

The next six families include the animals formerly known as Gymnosomatous Pteropods, characterized by the absence of mantle and shell, the reduction of the ventral surface of the foot, and the parapodial fins at the anterior end of the body. They are all pelagic.

Fam. 2.—Pneumonodermatidae. Pharynx evaginable, with suckers. Pneumonoderma. Dexiobranchaea. Spongiobranchaea. Schizobrachium.

Fam. 3.—Clionopsidae. No buccal appendages or suckers; a very long evaginable proboscis; a quadriradiate terminal branchia. Clionopsis.

Fam. 4.—Notobranchaeidae. Posterior branchia triradiate. Notobranchaea.

Fam. 5.—Thliptodontidae. Head very large, not marked off from the body; neither branchia nor suckers; fins situated near the middle of the body. Thliptodon.

Fig. 51.—Embryo of Cavolinia tridentata. (From Balfour, after Fol.)
a,	Anus.
f,	Median portion of the foot.
pn,	Pteropodial lobe of the foot.
h,	Heart.
i,	Intestine.
m,	Mouth.
ot,	Otocyst.
q,	Shell.
r,	Nephridium.
s,	Oesophagus.
σ,	Sac containing nutritive yolk.
mb,	Mantle-skirt.
mc,	Sub-pallial chamber.
Kn,	Contractile sinus.

Fig. 52.—Styliola acicula, Rang. sp. enlarged. (From Owen.)
C, C,	The wing-like lobes of the foot.
d,	Median fold of same.
e,	Copulatory organ.
h,	Pointed extremity of the shell.
i,	Anterior margin of the shell.
n,	Stomach.
o,	Liver.
u.	Hermaphrodite gonad.

Fam. 6.—Clionidae. No branchia of any kind; a short evaginable pharynx, bearing paired conical buccal appendages or “cephalocones.” Clione. Paraclione. Fowlerina.

Fam. 7.—Halopsychidae. No branchia; two long and branched buccal appendages. Halopsyche.

Tribe 3.—Pleurobranchomorpha. Two pairs of tentacles. Foot without parapodia; no pallial cavity, but always a single ctenidium situated on the right side between mantle and foot. Genital duct diaulic, without open seminal groove; male and female apertures contiguous. Visceral commissure short, tendency to concentration of all ganglia in dorsal side of oesophagus.

Fam. 1.—Tylodinidae. Shell external and conical; anterior tentacles form a frontal veil; ctenidium extending only over right side; a distinct osphradium. Tylodina.

Fam. 2.—Umbrellidae. Shell external, conical, much flattened; anterior tentacles very small, and situated with the mouth in a notch of the foot below the head; ctenidium very large. Umbrella.

Fam. 3.—Pleurobranchidae. Shell covered by mantle, or absent; anterior tentacles form a frontal veil; mantle contains spicules. Pleurobranchus. Berthella. Haliotinella. Oscanius, British. Oscaniella. Oscaniopsis. Pleurobranchaea.

Sub-order 2.—Nudibranchia. Shell absent in the adult; no ctenidium or osphradium. Body generally slug-like, and externally symmetrical. Visceral mass not marked off from the foot, except in Hedylidae. Dorsal respiratory appendages frequently present. Visceral commissure reduced; nervous system concentrated on dorsal side of oesophagus. Marine; generally carnivorous, and brightly coloured, affording many instances of protective resemblance.

Tribe 1.—Tritoniomorpha. Liver wholly or partially contained in the visceral mass. Anus lateral, on the right side. Usually two rows of ramified dorsal appendages. Genital duct diaulic; male and female apertures contiguous.

Fam. 1.—Tritoniidae. Anterior tentacles form a frontal veil; foot rather broad. Tritonia, British. Marionia.

Fam. 2.—Scyllaeidae. No anterior tentacles; dorsal appendages broad and foliaceous; foot very narrow; stomach with horny plates. Scyllaea, pelagic.

Fam. 3.—Phyllirhoidae. No anterior tentacles, and no dorsal appendages; body laterally compressed, transparent; pelagic. Phyllirhoë.

Fam. 4.—Tethyidae. Head broad, surrounded by a funnel-shaped velum or hood; no radula; dorsal appendages foliaceous. Tethys. Melibe.

Fam. 5.—Dendronotidae. Anterior tentacles forming a scalloped frontal veil; dorsal appendages and tentacles similarly ramified. Dendronotus. Campaspe.

Fam. 6.—Bornellidae. Dorsum furnished on either side with papillae, at the base of which are ramified appendages. Bornella.

Fam. 7.—Lomanotidae. Body flattened, the two dorsal borders prominent and foliaceous. Lomanotus, British.

Tribe 2.—Doridomorpha. Body externally symmetrical; anus median, posterior, and generally dorsal, surrounded by ramified pallial appendages, constituting a secondary branchia. Liver not ramified in the integuments. Genital duct triaulic. Spicules present in the mantle.

Fig. 53.—Halopsyche gaudichaudii, Soul. (From Owen.) Much enlarged; the body-wall removed.
a,	The mouth.
c,	The pteropodial lobes of the foot.
f,	The centrally-placed hind-foot.
d, l, e,	Three pairs of tentacle-like processes placed at the sides of the mouth, and developed (in all probability) from the fore-foot.
o′,	Anus.
y,	Genital pore.
k,	Retractor muscles.
o and p, The liver.
u, v, w, Genitalia.

Fig. 54.—Ancula cristata, one of the pygobranchiate Opisthobranchs (dorsal view). (From Gegenbaur, after Alder and Hancock.)
a,	Anus.
br,	Secondary branchia surrounding the anus.
t,	Cephalic tentacles. External to the branchia are seen ten club-like processes of the dorsal wall, these are the “cerata” which are characteristically developed in another suborder of Opisthobranchs.

Fam. 1.—Polyceratidae. A more or less prominent frontal veil; branchiae non-retractile. Euplocamus. Polycera, British. Thecacera, British. Aegirus, British. Plocamopherus. Palio. Crimora. Triopa, British. Triopella.