166 MORSE, CLASSIFICATION OF MOLLUSCA 



This latter group, combining the permanent inversion 

 of the sac-walls with the lophophoric arms, is the first ap- 

 proach to the Brachiopocla. No organ corresponding to a 

 heart has yet been discovered. In the Brachiopoda (Se- 

 ries I, B) we have a permanent invagination of the sac, and 

 the mouth, as in Terebratula, already occupies a position 

 some distance from the posterior edges of the overlapping 

 shells, and the brachial coils permanently occupy the space 

 thus made.* 



We have in this group a dorsal flexure of the intestine, 

 and a tendency to terminate as in the Polyzoa. In Lin- 

 gula it terminates posteriorly and at one side. . By the 

 permanent inversion of the sac, the mouth makes a great 

 advance toward the anterior pole. In Terebratula, Wald- 

 heimia, and allied genera, where the sac is very short and 

 swollen, and the brachial coils very large, the viscera are 

 crushed to the front, and the intestine, which is short and 

 simple, is nearly, bent upon itself, though still occupying a 

 median line. In Lingula, where we have a very long and 

 flat sac, the intestine is long, and has ample room for con- 

 volutions, but the anus, instead of terminating in a line 

 with the mouth, is thrown to one side, in consequence of 

 this excessive flatness of the sac. The heart will be found 

 on the outer bend of the intestine and actually on the ven- 

 tral side; the nerve occupying its homological position. 



(The manner in which I view the Brachiopoda, if true, 

 will entirely reverse the accepted poles of their structure. 

 What has been considered as dorsal, is here regarded as 

 ventral, and what has been considered as anterior, is here 

 regarded as posterior. Further remarks on this will be 

 made hereafter). 



Thus far the balance of structure has been thrown to 

 the posterior pole of the sac, and though we see a cephali- 

 zation, or concentration of the muscular system and vis- 

 cera, toward the anterior pole in Brachiopoda, yet that pole 

 being essentially closed, we have no function manifested 

 at that end, except the degradational one of adhesion. In 



* "Terebratulina caput-serpentes, aiid Crania anomala, projected 

 their cirri beyond the margin of the open valves, and moved them as 

 the Polyzoa move their oral tentacles, but in no instance were the arms 

 extended." Woodward's Treatise, p. 466. 



