250 httroduction to Animal Morphology. 



tissue, and are united at their base by a band of the 

 same material. A groove runs along the upper 

 surface of each, bordered by a double row of many 

 parallel sub-rigid bristles or filaments, each of which 

 is clothed with cilia ; three muscular bands lie under 

 the groove and attached to these bristles, which can 

 move them {a) outwards, widening the groove [b], in- 

 wards, narrowing it, or [c] downwards. These bristles 

 are contractile* and hollow, with calcareous supports 

 in Spirifer rostrata and Terebratula pectunculoides. 

 The arms themselves are traversed by large canals, 

 and are chiefly moved by the distension of these with 

 fluid, which enters from the body cavity, and is re- 

 tained by the action of circular basal fibres. Each 

 arm contains — ist, a chief canal beginning csecally at 

 its base; 2nd, a small afferent canal whereby the 

 bristles may be distended ; 3rd, an efferent canal ; 

 4th, a prolongation of the body cavity forming an arm 

 pouch ; and 5th, a canal at the base for the reception 

 of the calcareous loop support. Besides these are 

 numerous lacunas for blood. 



The muscles which move the valves of Fig. 32. 



the shell are: — ist. Two pair of occlusores, 

 anterior and posterior, passing from valve to 

 valve, tendinous medially in Rhynchonella 

 and Terebratula. 2nd. Two pair of diva- 

 ricatores from the ventral valve to the hinge 

 process of the dorsal, one chief and one 

 accessory pair ; these are the cardinal ^^^^ ^^ ^^ ^^^^^ 



muscles of some authors. 3rd. AdjUStor Waldheimia seen inf 

 T r- 11 1 section. 



ventralis, from the ventral valve to the 



peduncle. In Lingula there are three adjusters {a. ex- 



ternus, centralis, and posticus) ; of these two (external and 



* A respiratory function is assigned to these. 



