576 BRACHIOPODA. 



transversely from the dorsal to the ventral surface through the 

 body-cavity (Ma). 



In many forms the shell is perforated by minute pores, which 

 however are closed externally by the outer uncalcified layer of the 

 shell. They transmit tubular prolongations of the mantle. The body 

 is bilaterally symmetrical, excepting for the anus which in some forms 

 is on the right side, and is enclosed by the shell, of Avhich it only 

 occupies the posterior part ; it possesses the two large reduplications 

 of the integument, the two mantle lobes already mentioned, which are 

 applied to the inner surface of the shell. The edges of the mantle 

 are thickened and carry setae, which are regularly arranged and 

 secreted by the lining of pits of the skin. The mantle may also 

 produce within its own substance calcareous spicules, or a continuous 

 calcareous framework. 



FIG. 459. Diagram of a view of the left half of Cistella (Argiope) neapolitana, showing the 

 alimentary canal, the left nephridiuin, and the buccal groove. The animal is supposed 

 to have been removed from its shell, and to be cut in two by a median longitudinal incision, 

 f.o external, i.o internal opening of nephridium n ; d.m dorsal, v.m ventral lobe of the 

 mantle; I and c lip ; t tentacles ; m month ; st stalk. (Modified from Shipley.) 



The mouth is placed in a transversely directed groove, bounded 

 dorsally by a lip, and ventrally by a row of tentacles. In the 

 simplest cases (Ar<jiope, Cistella) this groove has a somewhat circular 

 course (Figs. 459, 460 a), and lies along the dorsal part of the anterior 

 wall of the body which is spread out on the dorsal valve. The 

 mouth is in the posterior part of it, and the anterior part of it is 

 bent backwards. In other forms the buccal groove does not form 

 a complete circle, but is incomplete anteriorly, and the two ends 

 so formed are coiled into a spiral, which is variously disposed in 

 different forms. Moreover, in these forms it is not united to the 

 dorsal Avail, but projects into the mantle-cavity. 



In Wallheimia (Figs. 460, 462) the buccal groove with its dorsal 

 lip and ventral row of tentacles passes outwards (j) on each side 

 of the mouth, and forwards towards the opening of the shell ; it then 

 bends on itself and passes backwards (f, h) on the ventral side of the 



