Introduction to Animal Morphology. 249 



the dorsal valve, and may arise from each side of a flat inner 

 hinge plate (\Yaklheimia), or from an internal longitudinal 

 median ridge running along the dorsal valve of the shell 

 (septum dorsal e*), as in Bouchard ia and Kraussia, or from 

 both (Terebratella, Morrisia), or from the inside of the valve 

 itself. The base of the loop on eaeh side is called the cms, 

 and often has a crural process or spur passing inwards, which 

 may join its fellow of the opposite side, forming a crural 

 . The loop itself may be short, down-directed, or may 

 bend again upwards and backwards, and its two limbs may 

 remain separate (and become spirally coiled as in the extinct 

 Spirifers), or may unite, forming a terminal bridge. 



The mantle is bilobed, full of lacunary blood spaces, 

 the chief agent in respiration, as no separate gills 

 exist. The mantle surface often contains calcareous 

 spicules, sometimes branched, or united into a crust. 

 Its free border is beset with separate or clustered, 

 long, transparent, ringed bristles! arising in special 

 glandular follicles, movable by muscular fibres at- 

 tached to them at their bases. The mantle consists 

 of an outer, netted, connective layer, an inner, homo- 

 geneous laver, and a central lacunary area, bounded 

 by a reduplication of the two laminse. Muscular fibres, 

 anterior and posterior parietal muscles, exist in it, 

 especially in Lingula, passing from one mantle-lobe 

 to the oth'-r. 



The arms are two, long, hollow, symmetrical pro- 



:u-n at rest folded \\iih 1-20 spinil windings, 



or S-furnn-il Morrisia , and plao-d one at cadi side 



of the mouth, attached to the loop of shell uhni it 



consist of semi-cartilag nn< ctive 



.11.1. 



