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MANUAL OF ZOOLOGY. 



of the shell, which is partitioned off by a membranous septum, 

 which is perforated by the aperture of the mouth. The 

 remainder of the cavity of the shell is almost filled by two 

 long oral processes, which are termed the 'arms,' and from 

 which the name of the class has been derived. These organs 

 are lateral prolongations of the margins of the mouth, usually 

 of great length, closely coiled up, and fringed on one side with 

 lateral processes, or 'cirri.' In many Brachiopods the arms 

 are supported upon a more or less complicated internal calca- 

 reous framework, or skeleton, which is sometimes called the 

 ' carriage- spring apparatus.' 



Fig. 75. Brachiopoda. Terebratula vitrea. 1. Showing the ciliated ' arms.' 

 2. Showing the shell with its loop. (After Woodward.) 



The mouth conducts by an oesophagus into a distinct 

 stomach, surrounded by a well developed, granular liver. 

 The intestine has a ' neural flexure,' and ' either ends blindly 

 in the middle line, or else terminates in a distinct anus between 

 the pallial lobes.' (Huxley.) 



Within the pallial lobes there is a remarkable system of 

 more or less branched tubes, anastomosing with one another, 

 and ending in caeca! extremities. This, which has been 

 termed by Huxley the ' atrial system,' communicates with the 

 peri visceral cavity by means of two or four organs which are 

 called ' pseudo-hearts,' and which were at one time supposed 

 to be true hearts. ' Each pseudo-heart is divided into a 

 narrow, elongated, external portion (the so-called " ventricle ") 

 which communicates, as Dr. Hancock has proved, by a small 

 apical aperture with the pallial cavity ; and a broad, funnel- 

 shaped, inner division (the so-called "auricle ") communicating, 

 on the one hand, by a constricted neck, with the so-called 

 " ventricle ; " and, on the other, by a wide, patent mouth with 

 a chamber which occupies most of the cavity of the body 

 proper, and sends more or less branched diverticula into the 

 pallial lobes.' (Huxley.) This system of the atrial canals 

 has been looked upon as a rudimentary respiratory apparatus ; 

 but its function is more probably to act as an excretory organ, 



