242 Dr. T. Williams on the Mechanism of Aquatic 



If function were expressed in numeric amount by the dimensions 

 of the organs, the physiologist would assign to this class of ani- 

 mals a high degree of respiration. Minute structure is a factor 

 in the estimate. The gills of the Lamellibranchiate mollusk are 

 singularly and peculiarly formed: they admit of comparison 

 in structural characters with no other organ found amongst 

 Invertebrate animals. The meaning of a part is an inference of 

 the intellect. When exact, it is founded upon a correct appre- 

 ciation of structure. A ' law ' is upraised upon the basis of par- 

 ticulars. Let the following difficult inquiry be conducted in rigid 

 compliance with this regulus philosophandi. Though abundant, 

 the elder literature upon this subject has bequeathed little that 

 is accurate and true. Baer* alludes in a special manner to the 

 pectinated character of the branchiae in the Lamellibranchiata ; 

 he illustrates his description by the gills of Mytilus. Meckel t 

 depicts and describes in general terms a comb-like structure in 

 the gills of Spondylus, Pecten, and Area. Cuvier's figures and 

 descriptions f delineate the same formation. In his valuable 

 notes, Siebold § describes the branchiae in Pectunculus, Mytilus, 

 Area, Pecten, Avicula, and Lithodomus as consisting of a system 

 of parallel vessels. In the text of his work, however, Siebold, 

 like Mr. Hancock, speaks of the trellis-like network of the 

 branchial structures. Among the older authors by whom allu- 

 sion is made to the pectinated arrangement of the branchial 

 vessels, the names of Bojanus, Treviranus, and Poli may be enu- 

 merated. 



The contributions of Mr. Hancock upon this subject are the 

 most recent, special, and distinguished ||. By this observer three 

 types of structure are recognized. They are thus defined in his 

 own language : — " There appear to be three distinct modifications 

 of gill-structure in the Lamellibranchiata. In the first the 

 laminae forming the gill-plate are composed of filaments either 

 free or only slightly united at distant intervals, as in Anomia 

 and Mytilus ; in the second they are formed by a simple vascular 

 network, as in Mya, Pholas, &c. ; and in the third the laminae of 

 the gill- plate are complicated by the addition of transverse plica 

 composed of minute reticulations of vessels, as in Chamostrea, 

 Myochama, Cochlodesma, &c. Other modifications may exist, 



* Meckel, Arch. 1830, p. 340. 



t Syst. der Vergleich. Anat. vi. p. 60. 



X Regne Animal, nouvelle edit., Mollusques, pi. 74. fig. 2 a. 



§ Anatomy of the Invertebrata, translated by Burnett, p. 21 1 . 



|| The excellent papers of Messrs. Alder and Hancock, to which repeated 

 reference is made in the text, will be found fin various Numbers of the 

 ' Annals and Magazine of Natural History' for the years 1852 and 1853. 

 To the attentive perusal of these valuable essays the student of the subject 

 is earnestly advised. 



