26 GILBERT C. BOURNE. 



structures are not calicoblasts, since von Heider, from his 

 observations on them in Astroides, has recently inferred that 

 the calcareous tissue is deposited in the form of crystals within 

 the calicoblasts, just as the spicules are formed within the sub- 

 stance of the skeletogenous cells in Alcyonaria and calcareous 

 sponges, and that the radial striation seen in these pretended 

 calicoblasts is due to the presence of minute crystal of carbo- 

 nate of lime. Now we know that animal tissues on the verge 

 of calcification are extremely resistant to the action of acids, 

 but in this case the crystalline form being assumed, that stage 

 would have been passed, and the crystals, if they were such, 

 would have been readily soluble in acids. Yet the acids which 

 dissolved the whole coralla of the specimens which von Heider 

 examined did not suffice to dissolve the minute crystals also. 

 It might have been inferred, one would have thought, that 

 whatever the striation was due to, it was not due to the pre- 

 sence of crystals of calcium carbonate. There is then nothing 

 to disprove von Koch's statement that the calcareous tissue is 

 elaborated by and secreted by the ectoderm (i. e. calicoblasts), 

 and that the Madreporarian skeleton differs wholly in this 

 respect from that of Alcyonaria. 



The mesogloea in Mussa is perfectly structureless ; I failed 

 to detect even a fibrillar arrangement in it, except were it is 

 drawn out into pleats for the attachment of the longitudinal 

 mesenterial muscles. In the upper part of the polyp, that part 

 which occupies the spaces between the enormously exsert septa, 

 the mesogloea is enormously developed, giving a considerable 

 amount of stiffness to the tissues in this region (fig. 4). 



The endoderm is of the normal character, and is packed, as 

 is usually the case in Actiniaria, with zooxanthellse. The cells 

 covering the mesenterial filaments are long and columnar, and 

 contain many nematocysts of different kinds; the one kind 

 small, similar to those found in the ectoderm, the other kind 

 large, measuring '005 mm. in length when the thread is not 

 ejected. 



Ova were present in my specimens in the normal position, 

 towards the lower extremity of the mesenteries, embedded in 



