Class Thaliacea Salps And Doliolids Fig 271ac

These transparent, jelly-like planktonic tunicates swarm at certain times, becoming so crowded that they exclude most other zooplankton from coastal waters. They swim by a sort of jet propulsion and have the mouth at one end of a barrel-shaped body and the atrial cavity and its excur-rent opening at the other. Rings of muscle, their number

Figure 27.1 Thaliacea (A-C) and Appendicularia (D). A1, Pyrosoma, section through colony; A2, zooid; B, Doliolum gegenbauri; C, Thalia democratic; C1,solitary vegetative zooid; C2, colonial sexual zooid; D, Oikopleura dioica. aa, atrial opening; ac, atrial cavity; ba, mouth; bo, light organ; cc, common cloacal cavity; dg, dorsal ganglion; dl, dorsal lamina; g, gill; lo, blood forming organ; lv, longitudinal vessel; in, intestine; s, stigmata; st, stomach; vg, vegetative stolon. (Figure: after Thompson 1948.)

Figure 27.1 Thaliacea (A-C) and Appendicularia (D). A1, Pyrosoma, section through colony; A2, zooid; B, Doliolum gegenbauri; C, Thalia democratic; C1,solitary vegetative zooid; C2, colonial sexual zooid; D, Oikopleura dioica. aa, atrial opening; ac, atrial cavity; ba, mouth; bo, light organ; cc, common cloacal cavity; dg, dorsal ganglion; dl, dorsal lamina; g, gill; lo, blood forming organ; lv, longitudinal vessel; in, intestine; s, stigmata; st, stomach; vg, vegetative stolon. (Figure: after Thompson 1948.)

and arrangement unique to each species, encircle the body. The largest known tunicate (colonies up to 20 m) is a Pyrosoma species, a genus also famous for bioluminescent organs that produce brilliant displays visible from space.

All Thaliacea have complex life histories of alternating sexual and asexual generations. In salps, colonies consisting of chains of connected sexual zooids are produced by cloning from a stolon in the solitary individuals of the alternating asexual generation. Fertilisation is internal and embryos are incubated in the sexual colonial zooids. Only purse salps (genus Pyrosoma) are exclusively colonial. In doliolids the life histories are even more complex than in salps, involving sexually produced larvae that metamorphose into asexual individuals. The latter multiply to form polymorphic complexes of gastrozooids and phorozooids that are eventually freed and from which the sexual gonozooids eventually detach.

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