82 INTRODUCTION TO CLASSIFICATION. 



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

 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 Lamellibranehiata 

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

 LUSCA, and the three classes which follow, viz., the Ascidioida, 

 Srachiopoda, 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 Lamettibranchiata and Odonto- 

 phora 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 nervous system is greatly simplified in all three 

 classes, consisting, in the Ascidioida and Polyzoa, of a single 

 ganglion, sending perhaps a commissural cord round the gullet. 

 In the Srachiopoda the chief ganglia, which appear to be the 

 homologues of the pedal ganglia of the higher mollusks, and 

 are connected by a circumoesophageal 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 conveniently denominated 

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

 aware there is only one exception, Appendicularia) 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. 



I cannot doubt, then, that the MoUuscoida form a natural 

 assemblage ; but, until the precise characters, if any exist, which 

 unite them with the Mottusca proper can be clearly defined, I 

 am inclined to think it might be better, as in the case of the 



