ACTINIARIA 

 102 



The ectoderm of the tentacles is high and contains numerous, small nematocysts 17 — 22 x 2 (2,5) n in size, 

 and very numerous spirocysts, 12 — 19 // long. The ectodermal muscles of the tentacles are weak. 



The ectoderm of the actinopharynx is thick, folded and provided with nematocysts 19 — 24 x 2 /i 

 in size, often a little curved, widened in the basal end, and with indistinct basal part to the spiral thread. 

 Besides these, there are, in addition to supporting cells, also mucus-cells here, considerably broader than 

 those of the siphonoglyphe. The ectoderm of the siphonoglyphe is thicker than in the other part of the acti- 

 nopharynx, and composed of supporting cells with long cilia. At the basis of the ectoderm there are rather 

 numerous, long, granulous glandular cells, which are in communication with the surface of the ectoderm 

 through a narrow duct. At the transition between the siphonoglyphe and the actinopharynx the ectoderm 

 is a little differentiated. The supporting cells carry stronger cilia here than in the other parts of the actino- 

 pharynx and the siphonoglyphe, and the nuclei commonly are more elongated, while they are round in the 

 other parts of the actinopharynx and the siphonoglyphe (PI. 2, fig. 13). Some cells of this zone might, how- 

 ever, be ordinary supporting cells, as round nuclei are also visible here and there. These strongly ciliated 

 ribbons are comparatively broad at the oral side, but gradually become narrower and seem to end at some 

 distance from the lower edge of the aboral prolongation. Probably these strongly ciUated parts are of some 

 particular physiological importance, as they form a boundary between the siphonoglyphe and the other 

 part of the actinopharynx. The mesogloea of the siphonoglyphe is considerably thinner than the ectoderm, 

 but thicker than that of the actinopharynx proper. The endoderm of the siphonoglyphe is strongly vacuo- 

 lated and very high in the exocoel-parts, in the actinopharynx proper lower and not so rich in vacuoles. A 

 distinct stratum of nerve-fibrillae with few nerve-cells and a weak longitudinal muscle-layer is present in 

 the ectoderm of the siphonoglyphe and the actinopharynx. The conchula with its hollow prominences is 

 built as the siphonoglyphe. The aboral prolongation of the siphonoglyphe also contains parts of the acti- 

 nopharynx itself. On both sides of the middle part of the prolongation, consisting of the siphonoglyphe, the 

 actinopharynx is namely continued, forming two, in proportion to the plane of the actinopharynx, per- 

 pendicular lamellae, the free edge of which are more or less strongly recurvated (figs. 132, 133). The middle 

 part of the prolongation is built as the siphonoglyphe in its upper part, wliile the ectoderm of the perpen- 

 dicular lamellae is longitudinally folded as in the actinopharynx, and of the same structure as that; the 

 ectoderm of the recurvated part is smooth, but does not seem to differ in structure from that of the folded part. 

 The boundary zone (fig. 132 a) and the folded part gradually become narrower aboraUy, and at last disappear 

 (fig. 133). The stratum of nerve-fibrillae, the ectodermal longitudinal muscles and the structure of the endo- 

 derm and the mesogloea agree with those of the actinopharynx. In the recur\-ated lamellae the endodermal 

 muscles seem to be longitudinal, I cannot, however, decide it with certainty as my material has fallen short. 

 Concerning the relation of the aboral prolongation to the filaments compare below! 



The number of mesenteries is that typical of Pcachia viz 6 pairs of perfect and 4 pairs of im- 

 perfect mesenteries, the latter in the lateral and ventro-lateral exocoels. Among the perfect mesenteries 

 the ventral directives are the stronger, the dorsal directives the weaker (always?). Below the actinopharynx 

 the inner part of the pennons of the directive pairs is strongly curved towards the endocoels, in the other 

 pairs towards the exocoels. The pennons of the perfect mesenteries are strong, with numerous high folds 



