80 ON CLASSIFICATION. 



Lamellibranchs have the three fundamental pairs of ganglia of 

 the Odontopliora, but they possess no trace of the odontophore. 

 Furthermore, they are all provided with bivalve external pallial 

 shells, the valves being right and left in relation to the body. 

 No shell of this kind is found in any of the Odoniopliora. 

 Almost all Lamellibranchs, lastly, have a pair of lamellar gills 

 on each side of the body, and all are provided with auriculate 

 hearts. No doubt the Odontophora and the Lamellibranchiata 

 properly form parts of one and the same sub-kingdom, Mol- 

 lusca, and the three classes which follow, viz., the Aseidioida, 

 BracMopoda, and Polyzoa, are usually included in the same sub- 

 kingdom. 



But the difficulty of framing a definition which shall include 

 the last-named classes with the Lamellibranchiata and Odontophora 

 is almost as great as in the parallel case of the Annuloida and 

 Annulosa ; while, on the other hand, the Ascidians, Brachiopods, 

 and Polyzoa exhibit many features in common. Thus the ner- 

 vous system is greatly simplified in all three classes, consisting, 

 in the Aseidioida and Polyzoa, of a single ganglion, sending 

 perhaps a commissural cord round the gullet. In the Brachiopoda 

 the chief ganglia, which appear to be the homologues of the pedal 

 ganglia of the higher mollusks, and are connected by a circuni- 

 cesophageal cord, are combined with accessory ganglia, but these 

 do not seem to be identifiable with the pedal or the parieto- 

 splanchnic ganglia. 



Again, the fact that the heart, when present, is of a simple 

 tubular, or saccular, character, and is devoid of any separation 

 into auricle and ventricle, constitutes a wide difference between 

 these three classes and the higher Mollusks. On the other 

 hand, these classes, which may be convenientlv denominated 

 Molluscolda, resemble one another in the fact (so far as I am 

 aware there is only one exception, Appendicular ia) that the 

 mouth is provided with ciliated tentacula, disposed in a circle, 

 or in a horse-shoe shape, or fringing long arms ; that it leads 

 into a large, and sometimes an exceedingly large, pharynx ; and 

 that in two of the three, at least, that system of cavities commu- 

 nicating with the exterior, which has been called the "atrial 

 system," is greatly developed. 



