PLATE IV. 



PO RITES ASTR.EOIDES Lamarck. 



Fig. 32. — Discal view of two enlarged polyps in which the tentacles are fully expanded; the radial extent of the 

 mesenteries can be seen through the transparent discal wall: a, P. divaricata; b, P. clavaria. 



Fig. 33. — An expanded polyp of P. clavaria in which most of the tentacles are introverted, two only being partly 

 expanded. 



Fig. 34. — Retracted polyp of P. clavaria with the tentacles and disk still exposed. 



Fig. 35. — Retracted polyps of /'. clavaria. In the two to the right the column wall is partly folded over the disk, but 

 admits of the tips of the tentacles and middle of the peristome being seen; the tentacles are wholly hidden 

 in the polyp to the left. 



Fig. 36. — P. aMrseoides. Vertical section through a portion of the column wall with two mesenteries attached. The 

 section was doubly stained with borax carmine and methyl blue; the yellow contents of the pigment cells 

 were unaffected, the nucleus alone taking up the carmine. The nature of the nearly circular body in 

 the middle is somewhat doubtful. Similar bodies occur in numbers within the tissues of some polyps, 

 but are absent from others; perhaps they are the reproductive sporogonia of the perforating algae, but 

 the connection with the algal filaments has not been traced. Between the two mesenteries is the 

 skeletotrophic covering of a septum, and in the space formerly occupied by the corallum are found certain 

 bodies which seem to represent the early stages in the growth of the algal filaments. The endodermal 

 epithelium of only one face of the mesentery to the right is represented. X 900. 



Fig. 37. — Isolated pigment cells from the column wall, with a group of supporting cells. X 900. 



Fig. 38. — Transverse section through a mesentery, a little below the point at which it becomes free from the 

 stomodseum. The peripheral end at this level is opposite a canal, ami therefore appears five from any 

 connection with the polypal wall. The centripetal end is tipped with a deeply staining tissue, closely 

 resembling the stomodseal ectoderm; as yet it can scarcely be regarded as a mesenterial filament. On 

 some of the complete mesenteries the filament is never developed beyond this stage, but on others it 

 becomes definitely rounded off from the mesenterial endoderm (cf. fig. 39). The endoderm is highly 

 glandular, and bears yellow pigment cells, zooxanthellse, and a few large nematocysts. The mesenterial 

 plaitings supporting the retractor muscle are very feeble. To the right of the mesentery is a transverse 

 section through the skeletotrophic tissue covering a palus. The endoderm is very glandular, like that of 

 the mesentery, the mesoglcea is extremely thin, and the calicoblast ectoderm at this level is a deeply 

 staining layer, with numerous nuclei and highly protoplasmic cells. X 400. 



Fig. 39. — Transverse section through an interseptal loculus, some distance below the stomodseal region. The loeulus 

 contains a single, well-developed mesentery. The calicoblast layer at the periphery is represented only 

 by granular matter, with small nuclei here and there. The skeletotrophic endoderm scarcely differs 

 from its condition in the upper regions (cf. fig. 36). The mesenterial endoderm is much less vacuolated 

 than above, and the filament is rounded off, and displays the usual histological details, except for the 

 occurrence of the irregular yellow pigment cells. X 400. 

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