244 Dr. T. Williams on the Mechanism of Aquatic 



confusion*. Nothing less than a rediscovery of these skeletal 

 parts, by which the branchial vessels are maintained in situ and 

 the whole tubular system preserved in shape, could suffice to 

 render complete and consistent the demonstration of the anatomy 

 of the Lamellibranchiate gills. 



With the eye stedfastly fixed on these parts, it is surprising 

 that M. Deshayes could see in them no meaning, could read in 

 them no purpose. In defining the outline and office of the inter- 

 lamellar water-tubes, it is matter of wonder that Mr. Hancock 

 did not suspect the mechanical necessity for a supporting appa- 

 ratus such as this, without which the water-tubes could not sus- 

 tain their patency or their form. Such is the history of progress 

 in all the manifold paths of scientific observation. Discovery 

 must literally be prefigured in the intellect of the thinking ob- 

 server. In the absence of the foreshadowing conception, wondrous 

 things in nature rendered manifest by accident are vacantly 

 gazed at, left unfathomed, and then forgotten, or mentioned 

 only as incidents or episodes in the drama. The merit which 

 belongs to rediscovery is too often withheld from its author. It 

 is morally, in equity, not less worthy of honour than the first 

 discovery. 



The law was formerly stated that the blood-channels in the 

 gills of the Lamellibranchiate mollusks occur always in every 

 species in form of straight, parallel, independent, non-commu- 

 nicating tubes, supported on the two opposite sides by hyaline 

 cartilages, generally membraniform and semicylindrically curved 

 (PI. VI. figs. 6 & 6 2 ; and PI. VII. fig. 15). These blood- 

 channels never reticulate. At the free border of the gill the 

 afferent channel returns into the efferent in a looping manner 

 (PI. VI. fig. 1 e, /). The efferent like the afferent channel 

 preserves its individuality from one border of the lamella to the 

 other (in, e). The blood-current, therefore, preserves unmixedly 

 its singularity and independence from the beginning to the end 

 of its branchial orbit (PI. VIII. figs. 23 & 24). This is a striking 

 and remarkable characteristic. It is a molluscan peculiarity. 

 Its prevalence in this class is universal. It stands in contrast 

 with the crustacean. The network plan is here the type. In 

 some species of Annelids the branchial vessels observe a straight, 

 parallel, looping mode of division. In the Annelid the blood 

 is coloured and non-corpuscular. In the mollusk it is replete 

 with globules. The blood- globules travel through the branchial 

 c bars ' in a single series, or two abreast. In the crustacean 

 the blood-channels are imparietal sinuses. In the mollusk each 



* The reader is requested to refer to fig. 352, article " Conehifera," in the 

 first volume of the ' Cyclopaedia of Anatomy and Physiology.' 



