Pelagic Glossophora with the median foot much reduced, bat with two large lateral pedal lobes (epipodia ?) developed into swimming organs. Visceral dome elongated and secondarily symmetrical, rarely coiled spirally; either naked or covered with a shell. No ctenidia; anus, generative and nephridial apertures placed anteriorly, and usually on the right side of the body. Her-maphrodite.
The bilateral symmetry of the Pteropoda is without doubt a symmetry secondarily acquired (p. 455), and in many points, both of development and anatomy, they closely resemble the Gastropoda. The foot is very small in size. In Clio it consists of two pro-meso-podial lobes, and a small meta-podium. The latter bears an operculum in Spirialis and Heterofusus (Limacinidae), and the Veliger of Cymbulia and Tiedemannia. The two swimming lobes are probably homologous with epipodia, and generally appear later than the other parts of the foot. In the Pteropoda with shells, or Thecosomata, they grow round the head, which is consequently not distinct in this order. In the Gymnosomata, where the adult has no shell, the head is furnished with two pairs of cephalic tentacles - an anterior pair, which is not represented in Thecosomata, and a posterior nuchal pair. The latter are eye-bearing in Gymnosomata, in the adult Creseis, and the young Tiedemannia and Spirialis among Thecosomata. The mantle aborts in Gymnosomata; it forms a deep fold on the ventral aspect in Thecosomata; but in the Limacinidae, with a spirally twisted visceral dome, the fold is dorsal in position. All Pteropoda are furnished with a larval shell, formed by the everted shell-gland. A secondary shell is added subsequently.
Both are thrown off in Gymnosomata and the Cymbulidae; the larval shell only in Hyaleidae; while both persist in Creseis (= Styliola). The Cymbulidae form a third persistent hyaline shell of cartilaginous consistency. In other Thecosomata the shell is also hyaline, and apparently highly calcified.
It is spirally coiled in Limacinidae, in other Thecosomata symmetrical. The larval shell of Cymbulidae, however, is spiral.
The epidermis is ciliated in Clio. There are cutaneous glands, pigment cells, and calcareous concretions in the integument. Tiedemarinia has a rich development of chromatophores.
The nervous system consists of the cerebral, pedal, pleural, and visceral ganglia. They are concentrated round the oesophagus, especially in Thecosomata. The otocysts are closely attached to the pedal ganglia. The structure of the eyes on the nuchal tentacles is unknown. A right osphra-dium appears to be present in Thecosomata, and in Pneumodermon among Gymnosomata is innervated in the typical way.
The oral aperture is armed with various tentacle-like processes, which are inserted in Gymnosomata within its margin. Clio has three pairs, the surface of which is covered with epithelial papillae, each furnished with a sense-cell, and giving exit to the ducts of unicellular glands, which nearly fill the central cavity of each process or 'cephaloconus.' They act as organs of adhesion in securing the prey. Pneumodermon has a pair of arms, which bear pedunculate muscular suckers on their oral faces. Tentaclelike processes are found near the mouth in some Thecosomata, but their structure does not appear to have been investigated. The oral cavity of Clio and Pneumodermon contains a pair of eversible sacs armed with chitinoid hooks. A pair of chitinoid jaws is generally present, in addition to the radula. Salivary glands are absent or rudimentary in Thecosomata. There is a long oesophagus, a stomach, and intestine. The anus is on the left (?) side in Hyaleidae, Cymbulia, and Tiedemannia, on the right in all other Pteropoda. A liver is well developed in Thecosomata, but in the Gymnosomata is represented only by caecal processes of the stomach walls. The auricle is not sharply marked off from the blood sinuses. Its aperture into the ventricle is guarded by valves.
The ventricle is well-developed, and gives off an aorta, which divides into an intestinal and cephalic branch. Its ultimate ramifications open into a system of irregular coelomic sinuses. The general surface of the body is respiratory in Gymnosomata. Pneumodermon, however, possesses three contractile and richly ciliated processes at the apex of the visceral dome, in and out of which the blood passes. A semilunar series of folds within the branchial cavity on the body wall of Hyalea, and a transverse shield-shaped ciliated area on the wall of the mantle fold in the same Pteropod and some other Thecosomata, have had a respiratory function assigned to them. The nephridium opens near the anus. It is saccular, and in some, probably in all instances, opens into the pericardium. Unicellular glands open into it in Clio, and in the Hyaleidae its walls are said to be spongy and opaque. There is an hermaphrodite gland, an hermaphrodite duct, which may be dilated near its termination, a receptaculum seminis, and albuminiparous gland. The hermaphrodite duct opens near to but in front of the anus. In Pneumodermon a copulatory organ lies within the aperture of the duct, but in other Pteropoda it lies at some distance in front of the aperture.
It is always in- and e-vaginable1.
The ova are laid in a mass of albumen, and are usually found floating on the surface of the sea. Clio attaches the mass to sea-weeds when confined in aquaria. Segmentation is unequal. The velum is bilobed and large, except in Hyaleidae. The larva of Gymnosomata (Clio, Pneumodermon) loses the velum and shell, and then passes into a second larval form provided with three zones of cilia girding the body - one anteriorly, in front of the foot; the other near the middle; the third posteriorly. The first atrophies early; the middle one has been observed in a nearly adult Clio, and the hindmost in a Pneumodermon.
The Pteropoda are exclusively pelagic and carnivorous. They are found in all seas, sometimes, e.g. Clio, in immense numbers. Fossil forms, all extinct, appear in Cambrian strata.
There are two orders:
(1) Thecosomata, with mantle fold and shell; with three living families, Hyaleidae, Cymbuliidae, and Limacinidae (Spinalis), and the fossil families Conula-riidae, Tentaculitidae, and Thecidae.
(2) Gymnosomata. Mantle fold and shell absent in adult; a second larval form with three ciliated bands; with the living families Clionidae, e. g. Clio (or Clione) borealis, Pneumodermonidae, and Pterocymodoceidae.
Keferstein, Bronn's Klass. und Ordn. des Thierreichs, iii. 2. Clio borealis. Wagner, 'Die Wirbellosen des Weissen Meeres,' Leipzig, 1885. Cymbulia, external shape and position in shell, Macdonald, P. R. S. xxxviii. 1884-5. On certain Gymnosomata, Boas, Z. A. viii. 1885.
Cephalic appendages of Gymnosomata, especially of Clio, Pelseneer, Q. J. M. xxv. 1885. Structure of suckers, Niemiec, Recueil Zool. Suisse, ii. 1885.
Histology of Pteropoda, etc, Paneth, A. M. A. xxiv. 1885.