258 I'KOCKKDINGS OF THK M AI.ACOLOGICAI, SOCIETY. 



complicated, almost spongy structure met with in Anodonta aud other 

 specialized forms. ^ 



The question then arises why there should be this remarkable 

 development of gill in the Pelecypoda when the other Classes of the 

 mollusca find their requirements amply mot by far simpler structures. 



The answer seems clearly that it is a matter of facility of respiration. 

 The majority of the gill-bearing Gastropoda and the less specialized 

 Pelecypoda live in waters that are constantly in a state of more or less 

 agitation, and where, consequently, entangled oxygen is comparatively 

 abundant ; whereas the bivalves that burrow do not get the water in 

 their lurking places so fully or so frequently aerated, and hence the 

 necessity for being able to extract proportionately more oxygen from 

 the water around them and the consequent development of the gill in 

 response to this demand. 



The fact, for instance, that Anodonta has developed such a 

 complicated gill-stiucture becomes intelligible when it is borne in 

 mind that it lives mostly in ponds or sluggish water, poor in oxygen, 

 and has, moreover, for six or eight months out of the twelve to 

 shelter within its gill-chamber hundreds of young, all like itself 

 consuming oxygen from the same limited supply. 



If this explanation be the right one, and complicated gill-structure 

 be a result of environment, rather than progressive development, it 

 may well happen that some of the groups of Pelecypoda founded on 

 these gill -structures, particularly the more specialized ones, may prove 

 to comprise forms that taxonomically are extremes of more than one 

 family, just as the slugs have been shown to be. On this point it 

 will l3e necessary to await further careful investigations of the type of 

 those begun by Mr. Bloomer on the anatomy of the British Solenida3, 

 of which the latest have been laid before this Society. 



All this tends to throw doubt on the taxonomic value of gill-structure 

 alofie for this group, and to lend greater weight to Dr. Dall's caveat, 

 echoed by Dr. Ilidewood, " that systems based on a single character 

 . . . are bound to prove unsatisfactory as our knowledge of 

 intermediate types advances; and that almost any group may have 

 among its members some which retain archaicisms longer than the rest 

 . . . Any permanent classification must necessarily be eclectic, 

 considering all characters, and distinguishing sufficiently between 

 genetic and adaptive features." ^ 



So thoroughly has the nervous system of the Mollusca been worked 

 at and described that, though much doubtless still remains to be done, 

 it is possible to get a comprehension of the Avhole, and here at once 

 a definite progressive development is traceable. In the earlier and 

 more archaic Gastropoda the nervous system is diffuse, the nerve 



' For interesting papers on the circulation of the water through the Pelecypod gill 

 and the part these currents play in conveying food to the mouth^of the animal, 

 see Wallengren, " Zur Biologic der Muscheln " : Lunds Univ. Arskrift., n.f., 

 Afd. ii, Bd. i, nos. 2 and 3 (1905). 



- Dall : Trans. Wagner Free Institute, vol. iii, p. 505. Eidewood : PhU. Trans., 

 ser. B, vol. cxcv, p. 185. 



