408 Dr. T. Williams on the Mechanism of Aquatic 



the same position in the branchial sac, that is, on the roof, and 

 distant from the rectum [h). One border of the leaf is strength- 

 ened by a penknife-shaped cartilage, such as that which will be 

 defined in the Pectinibranchiates. The breathing crypt in Sili- 

 quaria anguina is depicted by Philippi as having the same con- 

 figuration. The branchiae diff'er but triflingly from those of the 

 former genus. By this observer, these parts are sufficiently clearly 

 delineated in the two preceding Tubulibranchs to justify the 

 inference that the branchiae are formed precisely on the plan of 

 those of the Pectinibranchiata. So similar to that of the latter 

 order is the general cavity, so analogous the contained parts, so 

 identical in structure the branchiae, that it is difficult to conceive 

 why Vermetus should be called a Tubulibranch, and Buccinum a 

 Pectinibranch. In minute anatomy, the description which at 

 another time will be given of the branchias of the latter will 

 apply also to those of Vermetus. When species, whose vital 

 organs are so similarly constructed, are placed at opposite extremes 

 of an extended scale, the anatomist may well exclaim, — how arti- 

 ficial, partial, and arbitrary is the classification of the mere 

 naturalist ! 



The Branchiae of the Chitonida. 



A Chiton has a carapace like an isopod Crustacean, a dorsal 

 vessel like an Annelid, bilateral symmetrical reproductive viscera 

 like an acephalous MoUusk, a head and foot like a patelloid 

 Gasteropod, a posterior anus like the Fissurellidse, and branchiae 

 like those of the brachyurous Crustacea ! Such manifold affi- 

 nities at once unite and sever this odd group from several most 

 dissimilar classes. Measured by the standard of its branchial 

 organs alone, it deserves a higher rank than that accorded to it 

 by the side of the Patellidae. The anatomical position of these 

 organs nevertheless allies the Chiton to the Patella. In both, 

 they are placed in the furrow between the border of the foot 

 and the edge of the mantle. But in structure they are totally 

 unlike. Imagination may indeed construct ideal analogies. If 

 a branchial cone (fig. 2) were placed on either side of the anal 

 debouchure in Chiton and then simply enlarged without change 

 of figure, the branchiae of Fissurella (fig. 10) would be simu- 

 lated. They are organically diff'erent from those of Emarginula 

 (fig. 9) and Patella (fig. 5). In structural characters the gills 

 of the last two genera bear the same relation to each other 



establish a true and natural terminological system in zoology before the 

 real structm-e of animals is made known. The mere naturalist can never 

 find himself in a position to construct a consistent " terminology." This 

 task, so important to the progi-ess of knowledge, must be jointly under- 

 taken by the philosophic anatomist and the descriptive naturaUst. 



