•214 THE VOYAGE OF H.M.S. CHALLENGER. 



G Z. External opening of the sac of a retracted gastrozooid. 



0. Mouth of the gastrozooid. 



S. Gastric cavity lined in its upper part by large elongate ovoid gastric cells ; in its 



lower, by ordinary endoderm cells. 

 T. One of the tentacles of the zooid, of which a pair are seen in section. 

 M. Longitudinal muscular layer of the zooid. The muscles are continued down on to 



the four main ccenosarcal canals leading from the base of the zooid. 



E, Ectoderm layer of the zooid. 



C. Cavity of one of the four large canals into which the zooid cavity divides at its base 



in order to become continuous with the canal system of the ccenosarc. 

 This canal is here shown as cut open, and is seen to be lined with endo- 

 derm cells, the layer of which is continuous with that lining the zooid 

 cavity. 

 S'. The calcareous style, here introduced to show the position which it occupies within 

 the cavity of the zooid in the retracted condition of the latter. It is covered 

 by a layer of ectoderm, and the endoderm lining layer of the zooid cavity is 

 reflected over it. 



F, F. Walls of the sacs of the zooids. 



D Z, D Z. External openings of the sacs of two retracted dactylozooids, one of which 

 is very small, the other of the largest size occurring. 



B. Body cavity of the larger dactylozooid. In this zooid the ectoderm, E, is thrown 

 into a series of folds in the retracted condition of the zooid. It presents 

 on its outer surface a continuous layer of nematocysts. The zooid cavity 

 is lined by a thick layer of endoderm. The zooid is attached to the side 

 of the base of its containing sac, and is thus bent upon itself somewhat at 

 its lower region. 



R. Retractor muscles, continuations on to the main basal canal of the zooid of the 

 longitudinal muscular layer, which is seen in section in the upper portion 

 of the zooid. 



N, N. Nematophores. 



D. Surface layer of the ectoderm. 



G, G'. Male gonophores. Those on the right seen in complete section, that on 



the left with its sac only open, the generative masses being left intact. 

 In G a ripe male sac filled with mature spermatozoa is seen situate 

 nearest the surface of the coral, and beneath is an immature gonophore 

 with its centrally placed spadix of endoderm, The axis of the two 

 generative masses not lying in the same plane, the sjjadix is not seen 

 in the upper riper sac, which is not divided by the section through the 

 axis. 



