Respiration in Invertebrate Animals. 245 



vessel is elaborately carved and wonderfully protected (PL VI. 

 figs. 6& 6^). The difference is as essential in kind as it is con- 

 spicuous ; it may serve hereafter to establish the true direction of 

 equivocal affinities. Subtle analogies, like deeply hidden differ- 

 ences concealed amid the profoundest recesses of the organism, 

 are often more conclusive in disputative questions of specific 

 and generic relationships, than diversities or resemblances graven 

 prominently in the manner of the outward form. 



In now entering upon the narrative of the minute structure of 

 the gills in the conchiferous moUusks, it must be premised that 

 illustrative types only can be comprehended in the story. Spe- 

 cific varieties and modifications must be left to the specific in- 

 quiries of individual observers. There prevails, however, such a 

 remarkable uniformity in the architectural principle on which 

 the breathing organs in all Lamellibranchiate moUusks are con- 

 structed, that departures from the central plan never involve a 

 change of type. Such variations are apparent, not radical. 

 Though a concise description, aided by illustrations, may enable 

 the author to convey a readily intelligible statement of these 

 parts, the reader must not infer that his task has been easy or 

 his labour light. He has traversed dark and tangled contro- 

 versies. For long he could pilot his course by the magnetism 

 of no clearly-defined principle. Evidence conflicted, assertions 

 bewildered ; the subject was intricate, the clue of principle was 

 wanting. He would fain trust that the histoi-y which he is 

 about to wi'ite will transform a pre-existing chaos into the 

 cultivated scene of exact demonstration. 



The minute structure of the gills in the Conchifera may be 

 conveniently described under the heads severally of the consti- 

 tuent parts of which they are formed. 



1. The parallel bars or vessels forming the lamellse. 



2. The borders of the lamellse, {a) attached, {b) free. 



3. The transverse connective parts — intervascular, or inter- 

 vectal. 



4. The interlamellar water-tubes and the intra-tubular fi-ame- 

 work of support. 



5. The ciliary system of the gills. 



1. In the Acephalous mollusk the branchial vessel is sculp- 

 tured upon one essential plan. All deviations from this plan are 

 inessential varieties. So singularly do these blood-canals differ 

 from ordinary blood-vessels, that they will be henceforth de- 

 scribed under the name of " branchial bars" The word ' bar ' 

 implies, first, straightness, and secondly, rigidity, two properties 

 which belong to the branchial bars. The word ^ bar ' involves 

 the idea of separateness, individuality and independence — cha- 

 racters which apply to the branchial bars. Rigid bars arranged 



