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The Class Enteropneusta (acorn-worms) consists of about 75 species of solitary, unsegmented worm-like invertebrates. The body is fleshy, cylindrical and divided into three parts: the muscular contractile proboscis (prosome), the collar (mesosome) and the non-muscular trunk (metasome), each with coelomic compartments. The trunk has many gill slits. Enteropneusts are strictly marine and benthic (either sessile or sedentary). At least one enteropneust species has been reported swarming at the surface of shallow water, feeding on phytoplankton (Brusca and Brusca, 1990) — note that this species, Glandiceps hacksi (Marion, 1885) does not occur in the North Sea.

Reproduction and larval development
Asexual reproduction occurs in some enteropneusts; sexual reproduction may involve direct development or indirect development. Fertilised eggs develop into a planktonic ciliated embryo; in a very early stage of its development, the tripartite body plan is established. From this stage on, the species that produced yolky eggs develop directly into juvenile worms without a larval phase; in the species that produced non-yolky eggs (for instance Balanoglossus), the embryo develops into the characteristic tornaria larva [Tornaria larva] with ciliary locomotive bands resembling those of certain larvae of the Phylum Echinodermata. The larva soon elongates with the three body regions (prosome, mesosome and metasome) becoming externally apparent. Planktonic tornaria larva are initially lecitotrophic, that is non-feeding, but later on in the sixth stage (see below), they may be feeding.
In the larval development six hypothetical stages are recognised (named after various zoologists) summarised below and illustrated with the succesive stages of Balanoglossus clavigerus.
¥ Stage I Müller — newly hatched, with pre- and post-oral bands; barely distinguishable from echinoderm auricularia [[l][][f][/f]# B.clavigerus—I ].
¥ Stage II Heider — as Müller stage, with primary telotroch but devoid of primary lobes or saddles; barely distinguishable from echinoderm auricularia [[l][][f][/f]# B.clavigerus—II ].
¥ Stage III Metschnikoff — with developing or fully developed primary lobes and saddles [[l][][f][/f]# B.clavigerus—III ].
¥ Stage IV Krohn — with developing or fully developed secondary lobes and saddles and secondary telotroch. Of all stages, in this stage the maximum size is attained, 1.25-2.0 mm body length. At this stage the specific features of the tornaria larva can be identified with certainty [[l][][f][/f]# B.clavigerus—IV ].
¥ Stage V Spengel — with regression of secondary lobes and saddles and diminution in size [[l][][f][/f]# B.clavigerus—V ].
¥ Stage VI Agassiz — advanced regression of all ciliary tracts, elongated longitudinally, metamorphosis imminent [[l][][f][/f]# B.clavigerus—VI ].

Three enteropneust species are known from the North Sea, listed below with the name of the tornaria larvae and illustrated with the fifth stage (being larger than 1 mm).

¥ Glossobalanus sarniensis (Koehler, 1886)
larva previously known as Tornaria bournei [l][][f][/f]# T.bournei—V
¥ Glossobalanus marginatus (Meek, 1922)
larva previously known as Tornaria mielcki [l][][f][/f]# T.mielcki—V
¥ Balanoglossus clavigerus (Delle Chiaje, 1829)
larva known under the adult name [l][][f][/f]# B.clavigerus—V

Generally tornaria are not easily to be identified to species, mainly because the relation between adult and larval stages is poorly described and the tornaria development involves considerable but gradual changes in the appearance and form of the ciliary bands [Tornaria larva].

Size
Tornaria size ranges from 0.3 to 2.0 mm in height, depending on species and developmental stage.

[After Brusca and Brusca, 1990; Hayward and Ryland, 1995; Burdon-Jones, 1957; Todd and Laverack, 1991]

Class Enteropneusta