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Doliolida are relative small (up to 1 cm), free swimming pelagic tunicates belonging to the Class Thaliacea. The adult test is thin, typically barrel shaped with branchial and atrial apertures at each end of the body. Each aperture is provided with ten or twelve lobes.
The branchial wall contains many (8-200) transverse gill slits that are arranged in a fairly large band that begins near the dorsal nerve ganglion, looping backwards and curves forward again ventrally, ending at about the middle of the ventral side of the body. Thus, the gills occupy the posterior half or more of the body.
Characteristic are the loop-like muscle bands, nine in the oozooid stage and eight in the phorozooid and gonozooid stage. The muscles are denoted with M1 to M9. (In the trophozooid, there are about four muscle bands). The first and last muscle band form the sphincters to the apertures. The digestive tract is located at the ventral side, posterior of the gills. It comprises a short oesophagus, a stomach and an intestine, which may be or not coiled.
Doliolida are hermaphrodite, but gonads are only present in the gonozooid stage. The ovary is ventral behind the stomach; the testes originate near the anterior edge of the ovary, extending forward ventrally or at the left side of the ovary. The endostyle lies ventral and anterior to the gills; it produces mucus that is stretched along the pharynx by cilia, thus forming a net for filter feeding.

Reproduction
The life cycle shows an alternation of generations, between the asexual oozooid stage and the sexual protogynous hermaphrodite blastozooid stage; the latter develops by budding from the oozooid stolon. Compared with the other thaliaceans, the doliolid life cycle is much more complicated as it comprises several types of individuals for the blastozooid stage, and only one of these, the gonozooid, has gonads; the others are asexual.
The sexual gonozooids produce the eggs, that are shed into the claocal cavity to escape and develop free of the parent. The egg develops into a typical tunicate tadpole-like larva with a notochord. The larva develops in a menbranous house; after it comes free, it loosens the tail and develops into an oozooid.
The oozooid has at the position of M7 and M8 a dorsal projection, the cadophore, pointing to the rear. Within the parental cuticle is a ventral stolon from which the new zooids (blastozooids) are budding and migrate to the cadophore where they attach and develop further. In this stage the oozooid is called a nurse, and serves to produce and carry the developing buds; then the doliolid can be considered as a true colony.
The two lateral rows of buds on the cadophore develop as trophozooids (also called gastrozooids), that are the feeding individuals of the colony; they lack gonads, and the musculature is very reduced. They have greatly enlarged gills and a very wide pharyngeal opening. The median row of buds on the cadophore develops as phorozooids, that are later loosening and become free swimming individuals. They lack gonads but have eight muscle bands, the rest of the attachment stalk remains as a ventral peduncle. On the peduncle are so called probuds, that divide and give rise to buds that develop into free living hermaphrodite gonozooids. The gonozooids have eight muscle bands; the arrangement of the internal organs is similar as in the phorozooids, although a ventral process is lacking. The ovary of the gonozooid produces up to three eggs, that once fertilised are liberated into the cloacal cavity and from there into the free water. As gonozooids are the result from budding, they occur in greater concentrations than the other stages.
The internal organs of the nurse (except for the heart) gradually disintegrate, and the muscle bands become wider and hence the interspace very small. The body gets empty, nevertheless it lives on for quiet a while as a so-called old nurse.

Distribution
Doliolida are marine holoplanktonic animals, living in the epipelagic or upper mesopelagic (a few not well described species live considerably deeper). They are found in warm oceanic waters; both in number of species and concentrations most abundant in the tropics and subtropics (including the Mediterranean Sea). In the considered North Sea area, doliolids occur as visitors (invaded with warmer waters) from the open NE Atlantic Ocean and the Lusitanian stream. They survive only in offshore waters and are good indicators of offshore water movements. In our area, doliolids do not survive the winter. Regarding the travel time to reach the northern North Sea with the Lusitanian stream from the originating waters of the Mediterranean and the Gulf of Biscay, doliolids are most likely to occur in the North Sea during late summer and autumn.

Systematics
There are three families considered: the Doliolidae with four genera and some twenty species, the Doliopsoididae with one genus and two species, and the Doliopsidae (Anchinidae) with a single species. The genera and species of doliolids are identified as the phorozooid and gonozooid forms; only four oozooids have been described yet, corresponding to the genera Doliolum, Doliolina, Diololum, and Dolioletta (Godeaux, 1998).

Identification of stages

— Nine muscle bands or a continuous muscle sheet, dorsal cadophore present
—> oozooid
— Eight muscle bands, vestigeal ventral stalk present, gonads absent —> phorozooid
— Eight muscle bands, no outgrows from the test, gonads present —> gonozooid

[Adapted and modified from Berrill, 1950; Fraser, 1981; Godeaux et al., 1998]

The key to the doliolids of the North Sea starts at Page 461: Doliolida. The following species are included:

Order Doliolida
Family Doliolidae
Dolioletta gegenbauri
Doliolina muelleri
Doliolum denticulatum
Doliolum nationalis

Order Doliolida