Fig. 290.—Illustrations of Pteropoda Thecosomata: A, Limacina australis Eyd.; B, Cleodora cuspidata Bosc. (shell only); C, Cuvierina columnella Rang; D, Creseis virgula Rang; E, Clio balantium Rang; f, f, fins; l, liver; o, ovary; sh, shell. (After Souleyet.)
In Cavolinia (= Hyalaea, Fig. [5] B, p. 10) the shell consists of two plates, the ventral being convex, with one to three sharp spines at the posterior end, the dorsal flatter, without spines. The aperture is broad, contracted dorso-ventrally. Two long pointed prolongations of the mantle project from the lateral slits of the shell, and probably serve to balance the bulky body when swimming. Fins trilobed at the margin. Cleodora has only rudimentary lateral prolongations, fins bilobed, shell triangular, angles greatly produced, aperture very wide, dorsal side keeled. In Cuvierina the shell is straight, sub-cylindrical, with a median partition, slightly expanding towards the apex, which is truncated in the adult. The principal sub-genera of Clio are Creseis, which has an elongated sub-cylindrical shell, sometimes slightly curved, smooth or grooved; and Clio proper, in which the shell is long, angular, with a dorsal rib, apex (= embryonic shell) rounded, constricted. Styliola and Hyalocylix also belong to this group.
Fam. 3. Cymbuliidae.—Test (which is not homologous with the shell of other Thecosomata) slipper-shaped, cartilaginoid, simply a thickening of the mantle; embryo with a calcareous, spiral, operculate shell. Genera: Cymbulia, Cymbuliopsis, Gleba.
Three other families, Hyalithidae, Pterothecidae, and Conulariidae, from Palaeozoic strata, are generally added to the Thecosomata. All are fossil only, and it is doubtful whether they are really Molluscan. Pelseneer holds that no true fossil Pteropoda occur until the lower Tertiaries.
Section 2. Gymnosomata.—Mantle and shell absent in the adult, fins not connected by a lobe, no branchial chamber, head well developed, with two pairs of tentacles, eyes on the posterior pair; cerebral ganglia above the oesophagus; buccal cavity provided with a pair of protrusible “hook-sacs,” radula generally with 4 to 12 hooked laterals, central tooth triangular, jaw in one piece, composed of horny plates, no horny plates in stomach, anus on the right side.
The Gymnosomata are carnivorous, feeding on Thecosomata and other pelagic animals, being provided for this purpose with a formidable buccal armature of hook-sacs and suckers. The intestine, as usual in carnivorous groups, passes straight from the stomach to the anus; the fins are not attached to the head, but to the anterior part of the body. The larva has a straight shell, which disappears in the adult. About 21 species are known, belonging to 7 genera.
Fam. 1. Pneumodermatidae.—Animal fusiform, fins rather small, head prominent, anterior part of buccal cavity protrusible, with suckers on the ventral side, hook-sacs well marked; branchia on right side, skin soft, pigmented. Genera: Dexiobranchaea, no posterior gill, hook-sacs short; Spongiobranchaea, posterior gill circular; Pneumoderma, gill tetraradiate, hook-sacs long.
Fam. 2. Clionopsidae.—Body barrel-shaped, proboscis three times the length of the body, no buccal appendages; hook-sacs short, no lateral gill, posterior gill tetraradiate, skin not pigmented. Clionopsis is the single genus.
Fam. 3. Notobranchaeidae.—Body ovate, buccal appendages conical, no lateral gill, posterior gill with three radiating crests, skin pigmented. Notobranchaea is the single genus.