Page:EB1911 - Volume 11.djvu/539

the exception of the Aplustridae, Lophocercidae and Thecosomata, the head is devoid of tentacles, and its dorsal surface forms a digging disk or shield. The edges of the foot form parapodia, often transformed into fins. Posteriorly the mantle forms a large pallial lobe under the pallial aperture. Stomach generally provided with chitinous or calcified masticatory plates. Visceral commissure fairly long, except in Runcina, Lobiger and Thecosomata. Hermaphrodite genital aperture, connected with the penis by a ciliated groove, except in Actaeon, Lobiger and Cavolinia longirostris, in which the spermiduct is a closed tube. Animals either swim or burrow.

Fig. 46.
A,	Eolis papillosa (Lin.), dorsal view.
	a, b,	Posterior and anterior cephalic tentacles.
	c,	The dorsal “cerata.”
B,	Tethys leporina, dorsal view.
	a,	The cephalic hood.
	b,	Cephalic tentacles.
	c,	Neck.
	d,	Genital pore.
	e,	Anus.
	f,	Large cerata.
	g,	Smaller cerata.
	h,	Margin of the foot.
C,	Doris (Actinocyclus) tuberculatus (Cuv.), seen from the pedal surface.
	m,	Mouth.
	b,	Margin of the head.
	f,	Sole of the foot.
	sp,	The mantle-like epipodium.
D, E,	Dorsal and lateral view of Elysia (Actaeon) viridis.
	ep,	epipodial outgrowths. (After Keferstein.)

Fig. 47.—Enteric Canal of Eolis papillosa. (From Gegenbaur, after Alder and Hancock.)
ph,	Pharynx.
m,	Midgut, with its hepatic appendages h, all of which are not figured.
e,	Hind gut.
an,	Anus.

⁠

Fig. 48.—Central Nervous System of Fiona (one of the Nudibranchia), showing a tendency to fusion of the great ganglia. (From Gegenbaur, after Bergh.)
A,	Cerebral, pleural and visceral ganglia united.
B,	Pedal ganglion.
C,	Buccal ganglion.
D,	Oesophageal ganglion connected with, the Buccal.
a,	Nerve to superior cephalic tentacle.
b,	Nerves to inferior cephalic tentacles.
c,	Nerve to generative organs.
d,	Pedal nerve.
e,	Pedal commissure.
e′,	Visceral loop or commissure (?).

Fig. 49.—Cavolinia tridentata, Forsk. from the Mediterranean, magnified two diameters. (From Owen.)
a,	Mouth.
b,	Pair of cephalic tentacles.
C, C,	Pteropodial lobes of the foot.
d,	Median web connecting these.
e, e,	Processes of the mantle-skirt reflected over the surface of the shell.
g,	The shell enclosing the visceral hump.
h.	The median spine of the shell.

Fam. 1.—Actaeonidae. Cephalic shield bifid posteriorly; margins of foot slightly developed; genital duct diaulic; visceral commissure streptoneurous; shell thick, with prominent spire and elongated aperture; a horny operculum. Actaeon, British. Solidula. Tornatellaea, extinct. Adelactaeon. Bullina. Bullinula.

Fam. 2.—Ringiculidae. Cephalic disk enlarged anteriorly, forming an open tube posteriorly; shell external, thick, with prominent spire; no operculum. Ringicula. Pugnus.

Fam. 3.—Tornatinidae. Margins of foot not prominent; no radula; shell external, with inconspicuous spire. Tornatina, British. Retusa. Volvula.

Fam. 4.—Scaphandridae. Cephalic shield short, truncated posteriorly; eyes deeply embedded; three calcareous stomachal plates; shell external, with reduced spire. Scaphander, British. Atys. Smaragdinella. Cylichna, British. Amphisphyra, British.

Fam. 5.—Bullidae. Margins of foot well developed; eyes superficial; three chitinous stomachal plates; shell external, with reduced spire. Bulla, British. Haminea, British.

Fam. 6.—Aceratidae. Cephalic shield continuous with neck; twelve to fourteen stomachal plates; a posterior pallial filament passing through a notch in shell. Acera, British. Cylindrobulla. Volutella.

Fam. 7.—Aplustridae. Foot very broad; cephalic shield with four tentacles; shell external, thin, without prominent spire. Aplustrum. Hydatina. Micromelo.

Fam. 8.—Philinidae. Cephalic shield broad, thick and simple; shell wholly internal, thin, spire much reduced, aperture very large. Philine, British. Cryptophthalmus. Chelinodura. Phanerophthalmus. Colpodaspis, British. Colobocephalus.

Fam. 9.—Doridiidae. Cephalic shield ending posteriorly in a median point; shell internal, largely membranous; no radula or stomachal plates. Doridium. Navarchus.

Fam. 10.—Gastropteridae. Cephalic shield pointed behind; shell internal, chiefly membranous, with calcified nucleus, nautiloid; parapodia forming fins. Gastropteron.

Fam. 11.—Runcinidae. Cephalic shield continuous with dorsal integument; no shell; ctenidium projecting from mantle cavity. Runcina.

Fam. 12.—Lophocercidae. Shell external, globular or ovoid; foot elongated, parapodia separate from ventral surface; genital duct diaulic. Lobiger. Lophocercus.

Fig. 50.—Shell of Cavolinia tridentata, seen from the side.
f,	Postero-dorsal surface.
g,	Antero-ventral surface.
h,	Median dorsal spine.
i,	Mouth of the shell.

The next three families form the group formerly known as Thecosomatous Pteropods. They are all pelagic, the foot being entirely transformed into a pair of anterior fins; eyes are absent, and the nerve centres are concentrated on the ventral side of the oesophagus.

Fam. 13.—Limacinidae. Dextral animals, with shell coiled pseudo-sinistrally; operculum with sinistral spiral; pallial cavity dorsal. Limacina, British. Peraclis, ctenidium present.

Fam. 14.—Cymbuliidae. Adult without shell; a sub-epithelial pseudoconch formed by connective tissue; pallial cavity ventral. Cymbulia. Cymbuliopsis. Gleba. Desmopterus.

Fam. 15.—Cavoliniidae. Shell not coiled, symmetrical; pallial cavity ventral. Cavolinia. Clio. Cuvierina.

Tribe 2.—Aplysiomorpha. Shell more or less internal, much reduced or absent. Head bears two pairs of tentacles. Parapodia separate from ventral surface, and generally transformed into