Characteristics of Lamellibranchiata. xcv 



On the other hand, the Pteropoda are closely allied to the Benta- 

 Udae, the Solenoconchae of Lacaze Duthiers, or Frosopocepluda of 

 Keferstein ; and this aberrant order, though possessed of a ' tongue,' 

 of cephalic tentacles, and of epipodial lobes, as well as a foot proper, 

 has nevertheless been separated from the Odontophora, on account 

 of the many points of affinity which subsist between it and the 

 Lamellibranchiata. The most important of these appear to be, 

 the bilateral character of the organ of Bojanus, and of the gene- 

 rative gland; the absence of any accessory reproductive organs, 

 either glandular or intromittent ; the absence of sexual cong*ress 

 and the consequent extra-corporeal fertilization of the ova; and 

 the singleness of the larval velum. Each one of these points, 

 however, is reproduced either amongst the Cephalopoda or the 

 Gasteropoda, and the sum total of them therefore proves, not that 

 the Dentalidae ought to be dissociated from, but merely that the 

 Lamellibranchiata are rightly associated with the Odontophora, 

 as Mollusca proper, notwithstanding these points of degradation. 

 The organization of the Dentalidae is modified for their special habit 

 of living immersed, during the daylight at least, in the sand ; and 

 as many of the points of difference between them and the natatorial 

 Pteropoda may be explained by a reference to this peculiarity, whilst 

 such points of resemblance as the likeness of their larvae to those of 

 Fnumodermoii and Clione are of purely morphological value, the two 

 sets of animals may, as suggested by Mr. Huxley, be placed together 

 in a class which would have very generalized affinities on the one 

 hand to the Cephalopoda, and on the other to the non-odontophorous 

 Mollusca s. Lamellibranchiata. 



Class, Lamellibranchiata. 



Mollusca, which, while agreeing with the other three classes of 

 the Sub-division of Mollusca proper in the properties distinguishing- 

 it from the Sub-division of Molluscoidea, differ from them in having 

 a bivalve shell secreted by a medio-dorsally attached and bilaterally 

 symmetrical mantle, in the absence of odontophore, of salivary 

 glands, of stomato-gastric and sympathetic nerve-ganglia, in the 

 great size of the collars formed by the cerebro-pedal and cerebro- 

 visceral nerve-commissures, in the bilaterality of their reproductive 

 glands, and in the absence of any accessoiy reproductive organs. 



