XII BEACHIOPODA 413 



and so hide, not the foot, but the head segment. On the outer surface 

 of the mantle-folds the valves of the shell now make their appearance. 

 The first chaetae are very long ; they are of a provisional nature and 

 are soon shed, and the sacs containing them disappear, but in their 

 place there appear more numerous chaeta-sacs with short chaetae, 

 which persist through life. The head segment becomes less and less 

 prominent. The first ciliated tentacles of the lophophore make 

 their appearance at the sides of the head segment (Fig. 328). Others 

 subsequently appear so as to form a transversely oval ring surrounding 

 the mouth. A part of this ring extends up on to the base of the 

 dorsal mantle -lobe, but there is no doubt that the whole of the 



Fig. 327. — Two larvae of Terdratulina septentrioimlis just before and at the time 

 of fixation. (After Morse. ) 



A, free-swimming larva. B, larva at the moment of fixation, ap, apical tuft of cilia ; h, head 

 segment ; ma, mantle segment ; ped, foot segment ; v.m.f, area of the mantle segment which is about 

 to become the ventral mantle-fold and has lost its cilia. 



lophophore must be regarded as belonging to the head segment; 

 later the oval ring becomes produced at the corners which constitute 

 the adult " arms." The pores in the shell, so characteristic of 

 Brachiopoda, are already to be seen. These are caused by the out- 

 growth of blind tubes from the mantle coelom through the ectoderm 

 {coe.c, Fig. 329). 



AFFINITIES OF THE BEACHIOPODA 



It must be obvious to the reader that there is still a wide and 

 promising field for further investigation in the elucidation of the 

 organogeny of a Brachiopod. Conklin's investigations, which have 

 done more to clear up the subject than any other, were done, as we 

 have seen, on material collected and preserved for him, as an interlude 

 in his more serious work on MoUuscan and Tunicate Embryology. 



