482 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



structures which appear as processes of the niesoglfta. In sections the processes are striated 

 toward their free extremitv, which in methyl blue and in carmine always stains much more deeply 

 than the remaining mesoglcea. Their function would seem to be to bind the soft tissues to the 

 corallum. Where the insertion of a mesentery on the column wall is seen in longitudinal section 

 the processes appear as represented in tig. 95, PI. XIII. 



The nature and origin of the mesogloeal processes has Ijeen specially studied bj^ Bourne (1899). 

 He shows that they are formed from special ectodermal cells which he terms (/csnioci/tefi. These 

 take their origin from certain cells in the calicoblast layer, and become secondarily attached to tlie 

 mesoglcea. The processes thus formed may be known as desmoidal processes, though Bourne 

 employs the term desmocyti' for them, as well as for the cells liy which they are produced. 



The desmoidal processes may occur at any part of the outer skeletotrophic tissues, but arc 

 most numerous in areas along which the ti.ssues may have to withstand, as it were, the strain 

 of any muscular activity of the polyp. The attachment of the mesenteries to the skeletotrophic 

 tissues represents such areas, and here desmoidal processes usually occur in numbers. 



Also, as shown in tig. 67, PI. IX, thev are specially developed in colonies along the line of 

 separation of one polyp from those adjacent. It is o))vious that at the point def<. j.rr. the polyp 

 upon expansion will tend to raise the skeletal covering from its adherence to the edge of the 

 calice; hence to meet this the skeletotrophic tissues are provided with a special development 

 of desmoidal processes. However fully expanded a polyp may be, it is never al>le to detach 

 its basal wail from its adherence to the coi'allum. 



The whole manner of distribution of the desmoidal processes fully supports Fowler's (1899) 

 suggestion that the structures are special devices for maintaining the adherence of the polypal 

 tissues to the skeleton. Their purpose in many ways is comparable with that of the liga- 

 ments in the higher animals. They do not occur over the actively growing regions of poh'ps; 

 the skeletotrophic ectoderm is hei'e a continuous epithelium (PI. II, tig. 8). 



ECTODERM OR CALICOBLAST LAYER. 



The l)asal ectoderm or calicoblast layer is of much importance in studies of the morphologj' 

 of the Madreporaria. seeing that by it is produci'd the entin' skeleton, or coral as popularlj' 

 understood. The nature of the layer, and the mode of formation of the skeleton by it, have been 

 the subject of much controvei'sy, with which the names of Milne Edwards and Haime. A. R. 

 von Heider, G. von Koch. Miss Ogihie. and G. C. Bourne are associated. Bourne (1899) has 

 recently sunuuarized tht- various views as to the structure and formation of the skeleton, and 

 has made a very thorough study oi the process as it takes place in ditferent genera of Antliozo;i. 

 With regard to the Madreporarian skeleton he finds, with \'on Koch, that the calicoblast layer is 

 everywhere a simple epithelium, the cells rounded, colunuiiir, or fused together, and that the 

 calcareous matter is laid down wliolly external to the polyp. He thus difi'ers from von Heider 

 and Ogilvie, who concluded that tlic calicoblast ectoderm was a multilaminar layer, and that the 

 skeleton resulted from calcification within the cells. Thi> calicolilasts described by von Heider 

 are shown to correspond with the desmocytes of Bourne, and are not concerned with the 

 secretion of the sktdeton. 



The results from the jtresent study fully conKrni those of von Koch and Fowler as to the 

 unilaminar condition of the calicoblast layer and the ectoplastic formation of the skeleton. 

 L sually the layer is only well de\eloped within regions of active gi'owth, as toward the 

 uppermost part of the eorallites (PL 11, fig. 8), or aborally, where dissepiments arc in course 

 of formation (PI. X, tig. T3). Elsewhere the calicol)lasts form an extremely flattened layer; 

 in tS/'deraxfrfea, however, the structure is the same practically throughout the whole of the 

 skeletal area (PI. XXIV). As a rule desmocytes are wanting where the calicoblasts are well 

 developed, l)ut are plentiful where the cells are nonactive, especially along the line of attachment 

 of the mesenteries to the basal wall. 



In some instances {M(idr<'j:w/'<f. fig. 1*3) the skeletogenic ectoderm shows distinct cell 

 limitations, as in ordinary columnar epithelium; but generally these are lost, and the contents 

 are arranged in a contiiuious manner and largelv vacuolated. 



