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 Cavoliniatridentata. (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.