134 Miscellaneous. 



In the animals thus placed the first diflPerence that presents itself, 

 and that which has most caught the attention of ohservers, is this : 

 the Anodonta has its valves lateral ; the Terebratula has one of them 

 dorsal, the other abdominal. 



This difference, which appears very great, has not quite so much 

 importance as we should be inclined to attribute to it at first sight ; 

 we need only free it from the secondary conditions which surround it, 

 so as to see only the fundamental parts. Thus the greatly developed 

 and multiplied muscles have become longitudinal and symmetrical, 

 in consequence of the arrangement of the valves, and they have at- 

 tracted the attention of naturalists perhaps too specially, and led 

 them to neglect other more important organs. 



As the Brachiopod lives attached, a special locomotive organ would 

 be useless to it ; therefore its foot is aborted, and with it the corre- 

 sponding portion of the nervous system. Here, morphologically 

 speaking, we have a great and fundamental difi^erence, very different 

 from that presented by the position of the valves. 



On each side of the mouth of the Terebratula we find two long 

 fringed arms, rolled up in a spiral form, and accompanied by a mem- 

 branous lip ; these are the analogues of the labial vela of the Lamel- 

 libranchiata. The investigation of the nervous system justifies this 

 notion ; for there exist two small symmetrical ganglia which, with the 

 assistance of the long commissure uniting them, surround the oeso- 

 phagus like a collar, and furnish nerves to the arms, as in the 

 Lamellibranchiata the analogous ganglia furnish the nerves to the 

 labial vela. 



These first ganglia, which are difficult to discover, correspond with 

 the oesophageal ganglia of the other Mollusca ; they are united by 

 long connectives with the most highly developed and therefore most 

 evident nervous masses, which are found above the mouth, in the 

 median line, in the fold of the two lobes of the mantle. We know 

 that this last organ performs, in great part, the function of the organ 

 of respiration ; and as it receives its nerves from this last ganglionic 

 centre, this may be regarded as the analogue of the pallio-branchial 

 centre. 



As to the pedal ganglia, they do not exist, as the organ for which 

 they are necessary is wanting. 



The organs of Bojanus and those of reproduction open in the 

 Terebratula, as in the Anodonta, symmetrically outside and by the 

 side of the pallio-branchial nervous centre. Moreover, according to 

 the beautiful investigations of Mr. Hancock, the heart in the Brachi- 

 opoda is dorsal, which furnishes an additional feature of resemblance 

 between the two groups, for in this way the central organ of the 

 circulation is separated from the organs of Bojanus and the pallio- 

 branchial ganglia by the digestive tube. 



Lastly, in the Brachiopod, as in the Lamellibranchiate Acephalan, 

 the organs just referred to are repeated symmetrically on each side 

 of the median line of the body. 



Thus if we suppress in the Lamellibranchiate Acephalan the foot 

 and the pedal ganglia, there remains an organism having the greatest 



