488 MEMOIRS OF THE NATIONAL ACADE^NIY OF SCIENCES. 



space and fuse, without tlic presence of any intervening- .soft ti.ssues. Intermediate stages in the 

 formation of ind(Mitations of the skeletotrophic walls linino- the iiiterseptal spaces are presented 

 by corals in which the septa bear only oranulations. The complete skeletal fusion necessarily 

 leads to the piercing of the skeletotrophic tissues originally covering the granulations, and by 

 means of which the calcareous additions are made. The mesenteiy contained in the interse])tal 

 chamber is likewise perforated as a result of the skeletal growth across the chamber containing it. 



Much discussion has arisen as to the systematic value to be assigned a synapticulum, accord- 

 ing as it is compli^ted by the simple enlargement of two granulations, or l)y the intercalation of 

 one or more additional centers of growth. The former \vd\e been termed by Pratz (1882) 

 "False synapticula," and the latter "True synapticula." The polypal tissues themselves show no 

 distinction, accoixling as one method or the other is followed, and for discussion as to their 

 importance in skeletal morphology the works of von Koch (LSitti, p. 2.59), Ogilvie (189(5, p. 18-1), 

 and Vaughan (1900, p. 47) among others may be consulted. 



The genus S!<lr(iHti-:ri( is especially favorable for a study of the relationships of the polyp to 

 these characteristically Fiingid structures (Pis. XXII. XXIII). In the corallum of Sidi:mdi'xa 

 the synapticula are seen as vertical rows of short, thick, nearly circular bars, stretching from 

 one septum to another across the interseptal space, and mainly restricted to the peripheral 

 region of each corallite." Upon decalcification of a polyp the soft tissues remaining are found 

 to be made up of a large number of radiating vertical lamelhe. In the upper region these are 

 united centrallv, but are mostly free from one another below, and all terminate at aliout the same 

 level. The complete separation of the lamelhe results from the fact that in the lower region of 

 each corallite the septa extend all the way from the calicinal wall to the central columella, and, 

 uniting with the latter, wholly cut off one septal loculus from another. In its natural state 

 each lamella is separated ])asally from the lower part of the corallum by a delicate horizontal 

 dissepiment, exactly as in other corals, except that the dissepiment in its course may encounter the 

 synapticula. The lamelhe thus represent the interseptal polypal tissues freed by decalcification; 

 each consists of two lateral walls which lined adjacent septa, and above contains a single mesentery, 

 while below it is empty. 



A surface view of an isolated interseptal lamella, slightly enlarged, is represented on PI. 

 XXII, fig. 152. The edge to the right is central in regard to the polyp. For nearly the whole 

 of its length the lamella is Y)erforated toward its peripheral border by three rows of round or 

 oval apertures. Smaller lamelhv may bear only two rows of perforations, while again there 

 may be four more or less complete rows in some of the larger polyps. The apertures in the 

 lamellte represent the spaces which before decalcification were pierced by the skeletal synapticula, 

 and in each corallite the increase takes place above and centrally. 



Various sections of jjolyps of S. ddcfca are represented on PI. XXIII, and from these the 

 relations of the fully formed synapticulum to the polyp as a whole can be ascertained. In both 

 transverse and vertical sections each interseptal chamber appears as if composed of several 

 wholly distinct .segments, the skeletal matter which separates them representing the synapticula. 

 Some of the chambers are wholly or in part occupied by a mesentery, while others are 

 empty; in the few instances in which the section of a chamt)er does not include a perforation, 

 or rather a synapticuhun, the loculus is complete from center to periphery. A consideration 

 of the varied appearances which would result from sections at different levels through the 

 complete lamella (fig. 152) will aid in an understanding of the ai)pearances presented })y the 

 difb'rent interseptal chambers. The synapticula never wholly isolates any portion of the internal 

 cavity, though they must interfere with the effectiveness of the peripheral circulation. 



The sections further demonstrate that a gradual atrophy of the mesenteries takes place in 

 the lower and peripheral parts of the polyp as these become invaded by the synapticula. In the 

 uppermost polypal regions all the mesenteries are attach(>d to the column wall and disk, and six 

 pairs extend inwardly as far as the stomodsum ; but a little below the level of the stomod.eum 

 the peripheral attachment l)ecomes lost, while in the lowest sections no part of the me.senteries 

 whatever remains. 



«For excellent reprei^entatioiis of the syna])ticula in S!ihrai>lr;rn see Miss ()y;ilvie's paper, 1896, pp. 180-1S2. 



