THE ANATOMY OF THE MADREPORARIA. 247 



outwardly from the rest of the colony. In luxuriant growth and 

 budding, however, according to Heider, both skeletons and soft tissues 

 of adjacent polyps may fuse ; an observation interesting as probably 

 indicating the history of the formation of the coenosarc and coenen- 

 chyme which characterise many other forms. There is one correction 

 to be made in his work which, for the sake of future workers in this 

 field, ought to be mentioned here, namely, that in his PL III he 

 frequently figures as endodermal cells small spherical bodies with a 

 well-staining nucleus, which are zooxanthelke or symbiotic unicellular 

 Algae, living free in the ccelenteron in such numbers as often to 

 completely obscure the true endoderm, with which they, of course, 

 have no connection. While accepting v. Koch's theory as to the 

 origin of the theca from fusion of the septa, he differs from it in some 

 details, regarding the " sutures " as merely cracks artificially produced 

 in the corallum. Septa and tentacles both entocoelic and exocoelic ; 

 mesenteries and their muscles arranged as in Actinia. For further 

 details, which are very thoroughly worked out, his paper should be 

 consulted. One point of importance deserves mention : between the 

 corallum and the structureless mesoderm-lamella which overlies it 

 immediately, and was generally understood to secrete it, v. Heider 

 detected certain cells, for the most part scattered, but in some places 

 forming a definite layer. To these he gave the name calycoblasts, and 

 assigned the function of coral-secretion, with great justice, as later 

 researches proved, though their origin was a matter of doubt till 

 cleared up by v. Koch. 



The latter, in a paper on the development of Astroides calycidaris, 

 brought into notice the following facts (9). When first fixed, and before 

 the secretion of the skeleton has commenced, the embryo is plano- 

 convex, and its ectoderm may be divided into two regions, correspond- 

 ing to its surfaces, the plane disc of attachment, or basal ectoderm, 

 and the convex portion, or lateral ectoderm, the centre of which is 

 invaginated as the stomatodseum. The skeleton first appears as small 

 pellets of calcium carbonate lying between the basal ectoderm and the 

 foreign body to which the embryo is attached, and is therefore outside 

 the animal, and consequently the result of secretion by the ectoderm. As 

 the corallum is always described in text books as a product of the 

 mesoderm, this observation cannot be too strongly insisted upon. 

 These pellets become first a ring-shaped disc, then a complete disc 

 lying between the basal ectoderm and the foreign body to which the 



