Part II. — StichodactylincB and ZoantJiece. 159 



As noticed among the external characters, the ectoderm of the column-wall is 

 thrown into fine ridges and grooves. In section the former are shown to be sup- 

 ported by comparatively long mesogloeal processes, which may even become 

 branched (PI. xi., fig. 6). Numerous clear gland cells are present in the layer, along 

 with fewer granular gland cells ; many of the former are fixed with the mucus in the 

 act of streaming out. A very distinct though weak ectodermal musculature is 

 seen in transverse sections, more readily noticeable on the mesogloeal processes. 

 The mesogloea is thin for its whole length, and becomes even more so towards 

 the apex. The endodermal muscle is recognizable throughout the extent of the 

 column as a feeble layer, but the fibres become a little stronger towards the 

 apex, though no concentration which can be regarded as a special sphincter 

 muscle takes place. The mesogloea is thrown into very slight folds for its support, 

 and very fine fibrils pass from these towards the free surface of the endoderm, 

 where the nuclei and zooxanthellee are mostly concentrated. The commensal algse 

 are much more abundant in the distal regions of the polyps than proximally. 



The marginal and the disc tentacles display a similar structure. For the 

 greater part of the length of the stem the ectoderm contains many gland cells, 

 and its histology closely resembles that of the column-wall. Towards the free 

 termination an important modification take place ; the majority of the cells are no 

 longer broad and glandular, but elongated, narrow, and closely aggregated, while 

 deeply-staining nuclei are abundant. A peripheral zone is made up almost entirely 

 of large elongated cnidocysts, with a fine spiral thread inside. In the deeper parts 

 of the layer others are seen in different stages of development, and very distinct 

 fibrillar and nervous layers occur just outside the ectodermal muscle. The ecto- 

 dermal and endodermal musculature are both very feeble. 



Professor ^PMurrich (p. 48) found the tentacles in the Bahaman examples 

 characterized by the total absence of nematocysts, a condition at variance with 

 the Jamaican representatives, where both the marginal and disc tentacles are 

 crowded with nematocysts around their extremity, rather more so in the mar- 

 ginal than in the inner tentacles. In a very young specimen sectionized, 

 however, I was unable to discover any, even on maceration. 



The mesogloea of the tentacles is extremely thin, except proximally, where 

 it becomes broader and almost homogeneous. The ectodermal muscle is rather 

 strong around the base of the tentacles, but weakens distally. Numerous zoox- 

 anthellse are present in the thickened endoderm. 



The ectoderm of the disc contains very many, clear gland cells, and a few 

 narrow nematocysts, and, in places, large, granular gland cells; endodermal and 

 ectodermal musculatures on fine mesogloeal processes are also clearly indicated. 

 Zooxanthellse are abundant in the discal endoderm, while few nuclei are to be 

 seen in the mesogloea. 



2 A2 



