410 Dr. T. Williams on the Mechanism of Aquatic 



project at right angles secondary laminae {a, a, a), laid flatwise 

 the one upon the other from the base to the apex of the process. 

 The laminse (fig. 4 d; figs. 2 k,2» a,a,a) are largest at the root, 

 smallest and tapering at the further free end. Each leaf (fig. 4 d) 

 being attached only at one border, is capable of floating freely 

 in the water. This is a point of immense functional advantage. 

 The aerating current, however, is otherwise brought under mus- 

 cular control. The row of gills, being disposed in an angular 

 groove between two strongly contractile and extensile parts, 

 namely the edges of the foot and mantle, are mechanically 

 operated on by the current thus muscularly set in motion. Thus 

 the laminae are separated from one another. 



The gills of Chiton are much more parenchymatous or fleshy 

 than those of Patella. They possess an obvious power of col- 

 lapsing and expanding. Muscular fibres are disposed variously 

 throughout the entire gill. Even the borders of the leaflets are 

 contractile (fig. 4 d) . But the axial plane, in which the two large 

 vessels (fig. 4 a & 6) are lodged, is conspicuously fleshy and mus- 

 cular. By the fibres situated in this axis the whole process 

 may be shortened, and drawn strongly up towards the base. 

 This power may be given in order to protect the part, or to 

 quicken the circulation, or efiectually to cleanse the gill of efi'ete 

 water. Each branchial process carries in its central plane two 

 large vessels («, b) . They are connected with two main trunks 

 which run along the edge of the mantle. They are respectively 

 afferent and efferent. Thus far the apparatus is simple. The 

 circulation in the laminse is infinitely more complex (fig. 4). 

 From the main afferent vessel {a, arrow) of the branchial process 

 secondary branches (e) proceed. These latter run along the 

 attached border or root of each leaflet. A similar secondai-y 

 efferent vessel runs parallel to it on the other side of the same 

 border (e'). The two vessels are connected together by means 

 of the looped, parallel, ultimate blood-channels of the laminse 

 (c, c'). These latter are the true respiratory capillaries. They 

 form in the substance of the leaflet two layers of vessels. The up- 

 per loops into the lower layer at the free border {d) of the lamina. 

 Thus then the vital fluid flows, in horizontally parallel streamlets, 

 of extreme minuteness, along the upper aspect of a sheet, itself 

 flattened in the highest degree ; following the direction of the 

 loops, it curves round at the distal margin, returns in a similarly 

 distributed stratum along the inferior face of the lamina, and 

 reaches in the form of an arterial fluid the efferent vessel at the 

 fixed base. Although these ultimate blood- channels are un- 

 questionably separately walled conduits, they branch here and 

 there and unite with those in the neighbourhood. This branch- 

 ing however so seldom occurs, that each vessel may really be 



