114 ACTINIANS FROM THE BAHAMA ISLANDS 



quite conspicuous." I can add to this statement as to the distribution 

 of the tubercles the fact that the tubercles are more numerous and more 

 perfectly developed upon one side of the column than on the other, the 

 arrangement described by Haddon and Miss Shackleton ('91) for 

 I. asymmetricus being thus recalled. 



The column wall is very much thinner than that of /. tuberculatus, 

 and in addition does not show any of the large canals filled with cells, so 

 abundant in the Bermudan form. Numerous canaliculi and scattered 

 cells, the latter frequently placed in the canaliculi, occur, the mesogloea 

 being otherwise structureless. The ectoderm of the column presents 

 the arrangement which is usual in Zoanthids, being covered by a layer 

 of mesogloea, the subcuticula, from which trabeculse extend inwards 

 to unite with the general mesogloea, cavities being thus formed which 

 inclose packets of ectoderm cells (fig. 7). Externally to the subcuticula 

 is a delicate cuticula to which foreign particles adhere, and the ectoderm 

 cells contain numerous Zooxanthellae. This arrangement is identical 

 with what I have described for /. tuberculatus ('Sga), and agrees in its 

 essential particulars with the accounts of the majority of the authors 

 who have given the matter their attention. Quite recently Von Heider 

 ('95), in his account of Zoanthus chierchicB, has come to the conclusion 

 that the trabeculae which separate the various packets of ectoderm are 

 cells, and that the subcuticula is formed by the fusion of their outer 

 ends. Such an interpretation of the appearances is certainly not 

 applicable to either of the species of Isaurus I have examined ; the 

 trabeculae are so thick that their continuity with the mesogloea is be- 

 yond question, and, furthermore, the structure of the subcuticula shows 

 it to be identical with the matrix of the mesogloea. Nor do the 

 appearances presented by such a form as Zoanthus sociatus warrant 

 the belief that the trabeculae are otherwise constituted than in Isaurus, 

 and so far as my observations are concerned they tend to show that 

 the conclusions of Von Heider are erroneous. 



The tubercles are thickenings of the mesogloea, but each contains 

 also a cavity lined with cells continuous with the endoderm. Dr. 

 Northrop's notes contain the following description of the tubercles: 

 "In places the mesogloea contains on its inner side deep cavities which 

 are lined by the endodermis. The muscular layer of the mesogloea, 

 however, does not enter the cavity, but forms a ring around its base 

 {i.e. its mouth), so that when contraction takes place, the base of the 



