BRACHIOPODA. 



643 



sometimes united, but are more commonly distinct, and the repro- 

 ductive elements reach the exterior by means of two or four tubular 

 " nephridia," which open on the one hand into the body-cavity, and 

 on the other hand into the pallial chamber. The generative glands 

 are developed in the lobes of the mantle, and commonly leave im- 

 pressions on internal casts of the shell (fig. 488, o). The embryo 

 is ciliated and freely locomotive, but becomes fixed in the course of 

 its development. 



The shell of the Brachiopoda is essentially calcareous, but it may 

 be largely composed of horny matter (as in Discina), or the car- 

 bonate of lime may be largely replaced by phosphate (as in Lingida). 



c 



_____ 



B 



Fig. 489. — Minute structure of the shell of the Brachiopoda. A, Tangential section of the 

 shell of the recent Waldhehniaflavescens, greatly enlarged, showing the tubuli and the ends of 

 the flattened fibrous prisms of the shell ; b, Vertical section of the same, less highly magnified ; 

 c, Tangential section of the shell of Cyrtina Hamiltonensis, from the Devonian rocks of Canada, 

 greatly enlarged, the tubuli being mostly filled with peroxide of iron ; d, Vertical section of the 

 same, less highly magnified. (Original.) 



In the genus just mentioned, the shell consists of alternating layers 

 of horny and calcareous composition, the calcareous layers being 

 traversed by fine tubuli. As regards its microscopic structure, the 

 shell of the Brachiopods consists of " flattened prisms, of consider- 

 able length, arranged parallel to one another with great regularity, 

 and at a very acute angle — usually only about io° or 12 — with the 

 surfaces of the shell" (Carpenter). The oblique fibrous prisms 

 which compose the shell are best seen in vertical sections (fig. 

 489, b), but their irregular and closely dove-tailed extremities are 

 also shown in tangential sections (fig. 489, A and c). In some 



