Beroidae
Beroidae is a family of ctenophores or comb jellies more commonly referred to as the beroids. It is the only family within the monotypic order Beroida and the class Nuda. They are distinguished from other comb jellies by the complete absence of tentacles, in both juvenile and adult stages. Species of the family Beroidae are found in all the world's oceans and seas and are free-swimmers that form part of the plankton.
Some members of the diverse genus Beroe may occasionally attain a length of up to 30 centimetres (12 in), though most species and individuals are less than about 10 cm; Neis cordigera is among the largest species in the class, often exceeding 30 cm (12 in) in length.[1] The body is melon or cone-shaped with a wide mouth and pharynx and a capacious gastrovascular cavity. Many meridional canals branch off this and form a network of diverticulae in the mesogloea. There are no tentacles but there are a row of branched papillae, forming a figure of eight around the aboral tip.[2]
The sack-like body of the Beroe species may be cylindrical in cross section, or compressed to varying amounts according to species, while Neis is somewhat flattened and characterized by a pair of trailing gelatinous "wings" that extend beyond the aboral tip.
Like other comb jellies, the body wall of nudans consists of an outer epidermis and an inner gastrodermis, separated by a jelly-like mesoglea. The mesoglea has pigments that give many nudan species a slightly pink color; Neis cordigera may be yellowish or a deep orange-red.
Some nudans have a very large oral cavity, allowing them to swallow prey whole. While swimming, and particularly while pursuing prey, they close their mouths like a zipper so that they maintain a streamlined profile. The mouth is zipped closed from each end, and the edges seal shut by forming temporary inter-cellular connections. When close to the prey, the lips contract and the mouth is opened rapidly, sucking in the prey. This action is reversible and the lips can be resealed. An alternative method of feeding involves the lips spreading over the prey and the sword-shaped macrocilia lining the lips chopping off chunks.[3] The lips are sealed by the presence of adhesive strips of epithelial cells along their opposite edges. Not all species seem to have these strips which seem specifically adapted to those with wide mouths whereas species with small oral openings are able to control the opening of the aperture by more conventional means.[3]
Directly inside the mouth opening, in the lining of the gullet, can be found characteristic finger-like processes known as "macrocilia". These were first described by J.G.F. Will in 1844 and further investigated by George Adrian Horridge in 1965. He found they were complex structures composed of 2,000 to 3,000 filaments in a single, conical functional unit. Each macrocilium shows the typical eukaryote construction of nine external and two internal microtubules. The individual macrocilium is between 50 and 60 micrometres long and 6 to 10 micrometres thick, with the cilia bonded together in a hexagonal cross-sectional structure by permanent fibrils in three different planes. A system of tubules connects the basal bodies from which the macrocilia grow.[4]
