94 



OF CONCHOLOGY. 



The cirri of the brachiopods are tubular and composed of two 

 layers. The inner one is composed of slender, brittle, longi- 

 tudinal fibres of chitine ; the outer is membranous, very thin and 

 regularly imbricated, the imbrications perhaps indicating stages 

 of growth. In Liyigula anatina and Discina lamellosa these 

 bristles are covered with small short pointed setellce, irregularly 

 disposed upon the surface. These setellee resemble the thorns 

 upon a rose bush, being larger at the base, but the portion of 

 membrane upon which they are seated is sometimes constricted 

 beloAY the base, giving them a pedunculated appearance. They 

 are not articulated however as they might appear at first sight. 



Some of the cirri are flattened a little and the transverse im- 

 brications are closer than in some others. This may be due to 

 differences in rapidity of growth. 



When the cirri are boiled in caustic potash the outer mem- 

 brane, setellce, and most of the imbrications disappear entirely. 

 Nothing remains but the chitinous tube, which, deprived of its 

 envelope, splits readily into longitudinal fibres, or collapses. 



When the difference in their construction and the fact that 

 they are confined to the mantle edge and are not found upon 

 the peduncle or other parts of the body, are recognized, it be- 

 comes very evident that they are by no means " identical "' with 

 the setae of the worms. 



The brachiopods are invariably attached by permanent mus- 

 cles to bivalve shells, while the Annelids have no muscular con- ' 

 nection with their univalve tubes of mud or sand. The fact 

 that Lingula 'pyramidata forms a " sand-tube " about the pe- 

 duncle, though interesting, has no bearing on the subject, since 

 it is obvious that it cannot be formed in the same manner as the 

 worm-tube, with Avhich it might be compared, and does not bear 

 the same relation to the animal. 



The brachiopods are, as a rule, pedunculated and usually at- 

 tached by their peduncles to submarine objects ; Lingula forms 

 no exception ; though non-attached individuals are abundant, 

 others of the same species are permanently fixed to stones or 

 shells. 



None of the Annelids are pedunculated nor attached by the 

 integument to submarine objects. Plioronis and Orepina, larval 

 Sipunculoid worms, Avhich have been relied upon to bridge the 

 chasm between the Polyzoa and the Annelida, have a distinct 

 arterial and venous system, the latter, certainly, and the former, 

 probably, have a straight intestine terminating in a posterior 

 anus,* and in fact if the crown of ciliated tentacles be hidden, 



*See Strethill Wright, pp. 314-316, (Ed. new Philosoph. Jouru. Vol. 

 if, 1856,) and compare Johnstone loc. cit. p. 266. 



