Haddon and Shackleton — A Revision of the British Actinice. 615 



unusually numerous — they contain pigment granules. In E. paguriphilus they are 

 very dark ; often they have a yellowish colour, and are somewhat opaque. 



Zooxanthellse are present in the ectoderm of the three species of Isaurus which 

 have been microscojjically examined, and in many other species of the Brachy- 

 cneminse, but apparently not in all. We have not found them in any of the 

 Macrocneminse. 



Incrustations. — The incrustations which form such a characteristic feature of 

 this group of Actiniae are absent in the genera Zoanthus and Isaurus, though very 

 rarely a stray spicule, or grain of sand, may be entangled in the cuticlar layer of 

 these genera. 



With regard to the other genera, according to our experience, it appears that 

 certain species have a proclivity for a particular kind of incrustation. The 

 character of the incrustation must be conditioned by the precise habitat, i. e. whether 

 sand-grains are calcareous or siliceous, or, again, whether the bottom is sandy or 

 stony ; if sponges are abundant on a rocky bottom (as, for example, in Albany 

 Pass, Torres Straits), the forms will probably largely make use of sponge-spicules, 

 as in Parazoantha douglasi. The best example we have of apparent selection is' in 

 the case of Episoanthus incnistatus ; of this species we have cut specimens from 

 Norway, Shetland, West of Ireland, and N. E. America {^E. americanus, Verrill), and 

 in all cases we find the incrustations to be composed almost entirely of grains of 

 sand. In the single specimen we have been able to examine, of E. macintoshi, 

 from Shetland, the incrustations are almost entirely Foraminifera. In Norman's 

 type specimens of Parasoantha angidcoma^ from Shetland, the incrustations include 

 grains of sand, Foraminifera, and sponge spicules ; this holds good for the same 

 species from the West of Ireland, as well as for the other species (P. dixoni) from 

 the same district. 



The amount of incrustation also varies — for example, the species of Epizoanthus 

 are usually thickly incrusted, but in Episoanthus paguriphilus the incrustations are 

 very few in number. In Parazoanthus dichroicus there are very numerous incrus- 

 tations, but in P. axinellai they are sparse, and in P. dixoni there are still fewer. 



Mesogloea. — The mesogloeal ground substance is always homogeneous ; it is 

 penetrated by numerous minute cells, which are sometimes star-shaped, but more 

 frequently produced at each end into a long fibril which extends in a radial direc- 

 tion. Some of these fibrils are undoubtedly connected with the ectoderm, and 

 others with the endoderm (PI. lxiv., fig. 1)*; it is impossible to determine whether 

 some may not stretch right across the mesogloea. We have not been able to 

 satisfy ourselves of their presence in every case (ex. E. wrightii). 



* Plates Lxi. to LXIV. -will be found in the Memoir of these Transactions immediately succeeding this 

 one, viz. that on the Zoanthese of Torres Straits. They are frequently referred to in the present account 

 of the anatomy of the group. 



