514 



Forty- fifth Report on tre State Museum. 



curves and spirals into the anterior chamber of the valves. They 

 are two in number, one at each side of the mouth. 



These arms are substantially composed of a compact or 

 cartilaginous tissue which is perforated for its entire length by 

 several canals, to which reference has been made in the descrip- 

 tion of the circulating apparatus. The outer edge of the arms 

 bears the cirri, which are kept in motion partly by muscular 

 contraction and partly by the filling and emptying of the 

 brachial canals. At the base of the cirri is a loose membranous 

 expansion or flap, known as the brachial fold or lever. 



Fig. 140.— Transverse section 

 of arm of Hemitkyris psitta- 

 cea. a, b, wall of great 

 brachial canal; c, grooved 

 ridge; d, brachial fold; e, 

 bases of cirri; /, expanded 

 orifices of cirri, opening 

 into canal g. (Hancock.) 



Fig. 141.— Transverse section of arm of Lin- 

 gula anatina, a, central brachial canal; 6 ( 

 posterior canal; c, lacunes; rf, longitudinal* 

 muscle; e, brachial fold; /, bases of cirri. 

 (Hancock.) 



In a few of the living genera (Lingula and Rhynchonella) and 

 in a great number of the extinct forms (Spirifer, Athyris, Rhyn- 

 chospira, Rafinesquina, etc., etc., and probably Orthis) the arms 

 are coiled in long spiral cones, the extremities of which may be 

 directed outward (Spirifer, Athyris), inward (Rhynchonella) 

 Zygospira), dorsally (Atrypa), ventrally (Amphiolina) or the 

 spirals may be nearly in a vertical (Cyolospira) or a horizontal 

 (Lingula) plane; and the number of revolutions in the spiral 



66 



