192 J. E. Duerden — Jamaican Actiniaria : 



noticed that where the former are present in excessive amount, the latter are 

 absent, and. vice versa ; the two may, however, exist side by side in the same 

 species. The granules are recognized as very small spheroidal bodies of various 

 sizes, devoid of a nucleus and cell-wall, these being easily detected in the com- 

 mensal algse. 



Most of the tissues of P. Siviftii are densely loaded with bright yellow granules 

 of all sizes, but no zooxanthellse occur. The endoderm of P. dichroicus is also 

 stated to be richly pigmented, and no zooxanthellse are seen. The converse holds 

 in P. tunicans, the endoderm cells throughout contain an abundance of unicellular 

 algse, but pigment granules are practically absent ; P. monostichus and P. separa- 

 tes show an admixture of granules and zooxanthellse. In the latter species a 

 peculiar accumulation of brown pigment granules is found in the endoderm, about 

 midway along the width of the mesenteries, this being the only occurrence in the 

 polyp. 



Similar relationships of granules and zooxanthellse are afforded by other 

 families of Actiniaria. According to my observations pigment granules only are 

 present in Bunodes granidifera and B. Krebsii, while they are replaced by zooxan- 

 thellae in Aulactinia stettoides. Most Sagartidas contain zooxanthellse, but in 

 Sagartia nivea (Verrill), the substitution of granules has occurred. The latter 

 condition is also the case in Actinoporus elegans and in Corynactis myrcia already 

 referred to. 



It seems likely that in some cases the pigment granules may perform the same 

 function as the commensal algse — that of respiration. If this be so, we may per- 

 haps regard them as free chromo-plasts, aggregated in the other case within distinct 

 cells, the zooxanthellse. 



Although the amount and relative proportion of the inclusions vary, yet a 

 curious similarity in their nature holds throughout the genus. Fine sand-grains 

 and siliceous sponge spicules, with an occasional Radiolarian and Foraminiferal 

 test, are characteristic of each species. Carlgren found much the same in 

 Gerardia. 



Haddon and Shackleton note the inclusions to be fairly numerous in P. angui- 

 comus, and less so in P. axinellce and P. dixoni. Calcareous sand-grains predomi- 

 nate in P. tunicans, and sponge spicules in P. monostichus, while both are numerous 

 in P. separatus. In the two latter the majority of the spicules are similar to those 

 of the sponge with which the anemone is commensal. 



A certain selection in the disposition of the foreign inclusions is also observable. 

 Practically all the calcareous sand-grains of P. tunicans are limited to a narrow 

 zone around the boundary of the ectoderm and mesoglcea ; while the sponge spicules 

 are distributed throughout the middle layer, extending even to its inner boundary 

 (Pis. xiu. and xiv., fig. 4). Further, the spicules are most numerous in the capitular 



