412 Dr. T. Williams on the Mechanism of Aquatic 



every Crustacean the extreme blood-passages are equally exposed 

 on both sides of a single current to the action of the surround- 

 ing medium. Thus the area of exposure being the same, the 

 functional value of a Crustacean gill may be equal to that of a 

 Molluscan, albeit the circulatory system of the latter may be 

 incomparably more perfect and elaborate than that of the 

 former. A curious fact may here be mentioned, as connecting 

 the branchial operations with the position of the cloaca : that in 

 the Chitonidse the effete current of water flowing in the respi- 

 ratory groove between the edge of the mantle and that of the 

 foot sets backwards towards the anus — in the Patellidse it sets 

 forwards, towards the common position of the mouth and anus. 

 To recapitulate : it may be stated that in figure the gill of the 

 Chiton is the counterpart of that of the Crab ; in the ultimate 

 arrangement of its vessels it conforms with the type prevalent 

 throughout the brauchiferous Gasteropods ; in its fleshiness, the 

 denseness of its epithelium, and in the presence of minute fol- 

 licles on its surface, it allies itself with the branchiae of the 

 Nudibranchs. Nothing is more easy than to prove the presence 

 of epithelium over the entire surfaces of the laminae. The 

 waving of the cilia is visible throughout the whole extent of the 

 surface. The cilia which are situated on the free margins (fig. 4 d) 

 are much larger, — supported by correspondingly larger cells than 

 those distributed over the flat face. Opportunities will after- 

 wards occur for considering the question, whi/, in the organs of 

 branchial breathing, ciliary epithelium should almost always 

 clothe even the ultimate vessels, and why they should be as con- 

 stantly wanting on the corresponding parts of the pulmonary or 

 air-breathing series. 



Branchial System of the Patellida. 



It is proposed here to take the branchiae of the genus Patella 

 as the type of those of the remaining genera of this order. But 

 it will be shown that the Fissurellidse, Emarginulidse, and Halio- 

 tidffi, &c., differ strikingly both in the special and in the general 

 characters of the branchial system fi-om the Patellidae. Though 

 there may exist other features which in the judgement of the 

 malacologist may justify the marshalling of these several genera 

 under one and the same order, estimated by the branchial appa- 

 ratus, the Patellidse ought unquestionably to stand apart and 

 alone. The author is deeply persuaded that minute ultimate 

 histological questions will some day in the history of science 

 exercise a far more potent sway over the minds of classifiers 

 than they have hitherto done. Unity and uniformity reign with 

 greater constancy in the small than in the great productions of 



