84 Dr. T. Williams on the Mechanism of Aquatic 



cartilage wliich occupies the dorsal edge is curved at its point 

 in some species (tig. 12), so that it acquires, its root being the 

 fulcral point, all the resilient qiialities of a bow. In others it is 

 blade-shaped (fig. 7). It tends always to straighten itself. This 

 tendency is expended upon the Hat surface and the free margin 

 of the lainin?e, which are thus maintained in a tightened state, 

 like outstretched or expanded sheets. This is undoubtedly the 

 true purpose which this peculiar cartilage is intended to fulfil in 

 the gills of this order of INIollasks. Its existence has never yet 

 been suspected by anatomists. From the mechanical, lever-like 

 character of its office, it is evident that upon its duly regulated 

 action must depend the function of the entire leaflet. Without 

 it, a sheet of such surpassing delicacy as an individual branchial 

 lamina could not sustain the required vertical position. Without 

 some such contrivance the leaflets would be driven, crushed and 

 folded confusedly by every current and pressure. An elastic 

 apparatus, of inconceivable beauty and perfection, is realized in 

 these hyaline invisible and hidden parts. They hold, with a force 

 of immeasurable gentleness, the respiratory laminse at such a 

 degree of tenseness as best favours the transit of the water 

 between them, and of the blood throughout the extent of their 

 plane superficies. No crumpling or folding or confusion of any 

 kind can happen even in the relative position of structures of 

 such extreme tenuity and slenderness. And yet it has never 

 occurred to the curiosity of any one of the thousand obseiTcrs 

 by whom these organs have been witnessed, to catechise Nature 

 as to the mechanism by which such wonders, though minute, are 

 accomplished ! In organic workmanship, minuteness and per- 

 fection are often twin qualities of the same machinery ! These 

 cartilages are peculiar to the gills of the Pectinibranchiata, and 

 as the unfolding of details proceeds, it will be seen that they 

 undergo variations of size and shape, but never of relative posi- 

 tion, according to the diff"erences of families, genera, and even of 

 species. Into the branchial system of this large and important 

 order they are special importations, fidfiUing purposes of a 

 special nature. 



But the office of the border-cartilages is not restricted to the 

 end which has just been defined. They conduct and protect 

 the larger aff"erent and efferent blood-channels of the laminae 

 (tig. 3e). It is by thus transmitting a primary column of blood 



placed on the glass slip, floated in salt water if the specimen be marine in 

 habits, in fresh water if from a freshwater habitat, and then lightly 

 covered with a plate of thin glass. A few laminae at the same time are 

 detached by means of needles and torn up, in order that the objects may 

 be examined under different points of view. Various reagents are used in 

 the examination of the vessels, cartilages, muscles and fibres, &c. of the 

 organ. 



