OF CONCHOLOGY. 93 



The muscles of the Polyzoa are not striated ; in arrangement 

 they strongly recall those of SaJpa. On the other liand the 

 muscles of many of the Brachiopods are notable for their very 

 conspicuous and remarkahle striation. 



The peduncle of Lingula anatina consists of four layers ; an 

 outer horny layer, imperforate and smooth, or slightly villous 

 under a very high power, and transversely wrinkled by contrac- 

 tion into irregular annulations ; second, a gelatinous, homo- 

 geneous layer of pellucid matter, probably of the same essential 

 character as the outer layer, but softer ; third, a very thin, 

 delicate single layer of transverse muscular fibres ("delicate 

 membrane," of Hancock) the most worm-like of all the char- 

 acters of the group in question. But an exact parallel to the 

 three layers just described may be found in the siphons of 

 Lamellibranchs (for instance 3Ii/a arenaria) though the fourth 

 layer, which in the Brachiopods is composed of simple longitu- 

 dinal muscular fibres, is, in the other group, modified for the 

 special use for which it is designed.* A similar arrangement 

 of circular muscles is found in the higher Ascidians. 



Finally, we find one special character worthy of mention upon 

 which Mr. Morse has laid much stress, namely, the "bristles " 

 which fringe the mantle-edge in most of the brachiopods, and 

 which he has found " identical " with the setae of worms. 



These " bristles " or cirri in the brachiopods are very peculiar, 

 and unlike anything reported by all writers on worms, accessible 

 to me. They emerge one or two, or rarely more, from follicles 

 on the edge of the mantle, exactly as we find the bristles of 

 Acanthochites doing among the chitons. The bristles of Plaxi- 

 fliora., however, are not homologous, being more accurately re- 

 garded as dermal prolongations. But, while seated in follicles, 

 as among Chitons, the cirri of the brachiopods are less like 

 Chiton-setJB than the latter are like worm-bristles. The last- 

 mentioned are solid, f usually falciform, rarely jointed, and of 

 homogeneous consistency, quite insoluble in boiling caustic 

 potash. They are sometimes serrated, and often provided with 

 a hook ; and in the tubicolous Annelids are repeated with every 

 segment. The Chiton-setas resemble them in being solid, and 

 homogeneous, but they are more simple in form, shorter, and in 

 some species strengthened with a limy deposit. 



* It is not impossible that, to some extent, the peduncle may be 

 analogous to the siphon; especially as in Miiller's figure of an em- 

 bryonic Discina., a little organ below the mouth (which Morse compares 

 with an Annelidan operculum) seems from its position, at least, to be 

 analogous with the '" foot" of bivalves, while the peduncle occupies rela- 

 tively the same position that the siphon would. 



t See Johnstone, loc. cit. p. 79, et. seq. 



