By Jean Bouillon, Cinzia Gravili, Francesc Pagès, Josep-Maria Gili, Ferdinando Boero

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When experimentally removed, and under certain conditions, the interstitial cells can be regenerated from the epithelial tissues. An introduction to Hydrozoa FIG. 21. Histology of the hydroids, photography of a longitudinal histological section of an expanded hydranth of Halecium halecinum (Leptomedusae) showing the two endodermic regions of the column characteristic of the Haleciidae (after Bouillon, original). D = desmocyte; Di = diaphragm; GC = gastric cavity; GD = digestive part of the column; H = hypostome; Ht = hydrotheca; ND = non digestive part of the column; Te = tentacle; Th = hydrocladium perisarc.

Boero tentacles The tentacles are generally inserted on the umbrellar rim, but in the Narcomedusae, for instance, they are on the latero-dorsal exumbrellar face. The ectoderm of tentacles is formed by very flattened epithelio-muscular cells, and provided with longitudinal smooth muscular fibers, whose contraction leads to tentacle shortening. Solid tentacles are devoid of digestive or excretory inclusions, they present a «chordal» aspect, and they elongate by contraction of their smooth circular musculature.

Hypostomial spherulous gland cells: elongated cells containing numerous, small, regular, vacuoles each occupied by a well defined droplet of secretion formed by various enzymes, namely: leucine amminopeptidases, proteinases of trypsic type, amylases and sometimes acid and alkaline phosphatases. An introduction to Hydrozoa FIG. 22. Histology of the hydroids, photography of a longitudinal histological section of a contracted hydranth of Sertularella polyzonias, Leptomedusae, showing the abcauline gastric caecum, the ectodermal mantle and its roof (after Bouillon, original).

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