REPORT OF THE DIRECTOR I9IO I99 



, Of the two fundamental lines of specialization under epidermal 

 respiration outlined above, it is very clear that the second, that of 

 specialization at plate angles or along sutures, would be vastly the 

 more important. Centers of stereom formation, no matter how open 

 the texture, would offer more resistance to the respiratory process 

 than would their edges or those subtriangular spaces not as yet 

 closed in by the developing plates. The latter would be the line of 

 least resistance and while the less active respiration, which would 

 still take place through the plates themselves, might lead to interest- 

 ing specializations of plate structure, the more promising field of 

 specialization at plate angles is the only epidermal form w^iich will 

 be considered in this paper. 



Synthetically again or by deduction we ma}^ postulate two 

 avenues of escape from the inimical influence of plate extension. 

 Either by invagination or evagination of ectodermal tissues, involv- 

 ing in either case some portions of the mesoderm, an increase of 

 respiratory area could be secured and so specialized as to easily 

 maintain the physiological balance. Plate extension would protect 

 and modify invaginated respiratory sacs and soon leave them com- 

 municating with the exterior only through small pores or narrow 

 slitlike openings. Either form of external orifice might maintain a 

 position on the suture and, repeating the process as the suture 

 lengthened, give rise to a linear series of such openings, the number 

 being dependent in part on the amount of plate extension. If the 

 external orifice of either form should become surrounded by the 

 stereom of a single plate, the opening would thereafter maintain a 

 fixed distance from the early center of the plate and a repetition of 

 the process would soon more or less fill the plate with such pores 

 or slits and make it appear at first sight as if wx were dealing with 

 a case of direct specialization of the plate stereom. 



It should also be borne in mind that the extension of plate stereom 

 might divide the external opening and would undoubtedly often do 

 so. If the water exchange was maintained in any degree through 

 ciliary action, any such variation would very materially heighten the 

 value of the sac for respiratory purposes. If both openings should 

 become inclosed by the extending border of one plate we should 

 have a structure very similar to a diplopore. If on the other hand 

 one of the openings should become inclosed by the stereom of one 

 plate and the other opening similarly inclosed by the plate across the 

 suture, the sac would become elongated with the growth of the plates 

 and a structure apparently similar to that presented by some pec- 



