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Taxonomische classification
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(Forskål, 1775)

Description
— Aggregate form with anterior end of body rounded, the posterior end is somewhat fusiform. The test is bluntly pentagonal and never echinate. M1-M3 and M4-M5 are fused mid-dorsally; both groups of muscle bands are separated in lateral view. The nucleus has a posterior projection. The test is pentagonal posteriorly. The number of muscle fibres in M1-M5 is 15-17, normally 16. The atrial opening is not central and there is a posterior lateral protuberance on one side only.
— Solitary form with smooth test, atrial palps simple and straight; lateral projections small, medioventral projections are small and of unequal length, the anterior ones are the smallest. All test projections are echinate. M1-M4 are continuous dorsally and ventrally; M5 is narrowly interupted ventrally. M2-M4 and M5-M6 are converged mid-dorsally over a short distance only. The stomach partly extends into the middle posterior projection.

Size
Length of aggregate zooids 1.8-18.2 mm. Length of solitary zooids 2.3-11.7 mm.

Depth range
Epipelagic, living in the upper 100 m, does not perform diurnal vertical migration.

Distribution in the North Sea
Possible occurring in the northern North Sea as a seasonal visitor.

World distribution
Cosmopolitan, eurythermic species from warmer waters; roughly occurring between ca. 60°N and ca. 40°S. Also present in the Mediterranean Sea.

Remark
In solitary zooids, the number of muscle fibres in M1-M6 shows clinal variation, decreasing from higher to lower latitudes, in other words, from colder to warmer waters (see Van Soest, 1975b). Shoals of T. democratica occurring off the Hebrides in July-August were reported by Sewell, 1953.

Thalia democratica