Introduction to Animal Morphology. 249 



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

 hinge plate (Waldheimia), or from an internal longitudinal 

 median ridge running along the dorsal valve of the shell 

 (septum dorsale*), as in Bouchardia and Kraussia, or from 

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

 itself. The base of the loop on each 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 

 bridge. 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 isbilobed, 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 bristlesf 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 layer, and a central lacunary area, bounded 

 by a reduplication of the two laminae. Muscular fibres, 

 anterior and posterior parietal muscles, exist in it, 

 especially in Lingula, passing from one mantle-lobe 

 to the other. 



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

 cesses, when at rest folded with 1-20 spiral windings, 

 or S-formed (Morrisia), and placed one at each side 

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

 exists. They consist of semi-cartilaginous connective 



* Double in Pentamerus. 

 t Branching in Discina. 



