38 NATURAL SCIENCE. jdly, 



monata) ; for Actaon is streptoneurous, that is to say, possesses a 

 nervous system in which the loop-Hke visceral commissure is crossed 

 over in the form of a figure 8, and hence shows that the Euthyneura, 

 in which the visceral commissure forms a simple loop, were in the first 

 instance streptoneurous like the Prosobranchiata, and that their 

 euthyneurity, if the word may be permitted, is the result of secondary 

 displacement of the visceral hump from left to right. This retrograde 

 displacement may possibly have been occasioned through the 

 hermaphroditism of the animal, since the Streptoneura are all uni- 

 sexual. Among the Streptoneura the Trochoid Rhipidoglossa are 

 nearest to Acto'oii. 



Another point of importance is the direct derivation of the 

 Cephalopoda from the primitive stock before the other branches 

 lead off. In thus opposing the Cephalopoda to the rest of the 

 molluscan phylum, Pelseneer agrees, as he himself is careful to point 

 out, with Giard, who advanced this view in 1876 (18). The grounds 

 for this theory are to be found, not only in the anatomy, but also in 

 the high antiquity of the class. 



As to the source whence the MoUusca arose, this, Dr. Pelseneer 

 thinks, must be sought in the Polychaeta Errantia, or that group of the 

 Worms to which, among others, the gaudy sea-mouse (Aphrodite) 

 belongs ; and, moreover, he is of opinion that the family Eunicidae 

 are, among the living representatives of that sub-order, the nearest, 

 in the ensemble of their organisation, to the MoUusca. 



Granting this conclusion, what then of the shell ? We are 

 tempted to speculate whether the power of specially secreting a 

 calcareous covering may not be the revival of a dormant faculty 

 (seeing that it is present in a collateral sub-order, the Tubicolae, or 

 Tube-building Worms), just as the individual offspring of higher 

 creatures will at times present characters or display capabilities that 

 have lain dormant in their immediate progenitors — an occurrence to 

 which the term atavism has been applied. 



REFERENCES. 



Moynier de Yillepoix, R. — Recherches sur la Formation et I'Accroissement 



de la Coquille des Mollusques [Thesis] . 8vo, Paris, 1893, pp. 158, 4 pis. 



(Originally issued in Joiirii. Anat. Physiol. Paris, ann. xxviii. (1892), pp. 461- 



518, 582-674, pis. xix., XX., xxii., and xxiii.) 

 lionge and Mer. — De la formation de la Coquille dans les Helix. Compies 



Rcndiis Acad. Sci. Paris, xc. (1880), pp. 882-5. 

 Gray, J. E. — Some Observations on the Economy of Molluscous Animals, and 



on the structure of their Shells. Phil. Trans., 1833, pp. 771-819. 

 Bournon, Comte de. — Traite complet de la Chaux Carbonatee et de 



I'Arragonite. 4to, Londres, 1808, torn, i., pp. 310-49, pi. i. 

 Fischer, H. — Recherches sur la Morphologie du Foie des Gasteropodes. 



Bnll. Scicnt. France et Belgique, xxiv., pp. 260-346, pis. ix.-xv. An abstract 



from Author's advance copy appears in Journ. R. Micro. Soc, 1892, pp 



596-7. 



