413 I)r. T. Williams on the Mechanism of Aquatic 



nature, and the philosophic anatomist may inderd well abandon 

 his studies in despair. In conclusion it should be remembered, 

 that between a single leaflet of the gill of Patella and a single 

 lamina of the same form andfigwe, taken from any small bra- 

 chyurous Crustacean, the extreraest difference exists. The Crus- 

 tacean gill is far less perfect, more rude in every sense, as a 

 purposive machinery. CaiTying a single stream of blood, whose 

 corpuscles are considerably larger than those of Patella, and 

 which runs in irregular passages lying between two sheets of 

 epithelium tied together only by means of accidentally distri- 

 buted islets of fleshy tissue, it contrasts strikingly with the 

 double-vessel system with the contractile and highly ciliated 

 elements of which the Patellan gill is woven. 



For reasons derived from other sources than those of the 

 branchiae, the genera Emarginula, Fissurella and Haliotis are 

 placed by malacologists in the Patelloid group. The branchise 

 of Emarginula (fig. 9 a, a) approach much nearer than those of 

 Fissurella (fig. 10 a, a) to the type of these organs as they exist 

 in Patella. The gills of Emarginula are attached to the base of 

 the cervical cavity (fig. 9 d) on either side of the outlet of the 

 intestine. They project /or?^'«rc?5 in form of tapering processes 

 {a, a) on either side of the head of the animal. They are chiefly 

 fixed at the base, but a slender membranous frenum, proceeding 

 from the median line of each, connects them to that portion of 

 the mantle which covers the roof of the cavity of the shell in 

 which the head and rectum are situated. They are thus held in 

 one position. They possess notwithstanding a slight power of 

 expanding and collapsing. They are foliated only on one side 

 (a, a) of a base. They are more comb-like than plume-like. In 

 this particular of configuration they approach closely to the 

 branchise of Patella. If the latter were seated on a free instead 

 of a fixed base^ the resemblance would be very near. Occupying 

 a position on either side of the notch [b) in the shell, the latter 

 should be regarded as playing as important a part in providing 

 a fresh carrent of water for the branchiae, as in conveying away 

 the excreta. ^ jiu y 



The ciliary mov^ihents observable in this region both in 

 Emarginula and Fissurella point to the same inference. Ex- 

 amined separately under the microscope, a single leaf taken 

 from the gill of Emarginula reticulata presents an outline not 

 unlike that of Patella. It is, however, more lancet-shaped. 

 In ultimate minute structure it corresponds exactly with the 

 aptellidan model already figured. Its vessels run in parallel 

 columns, looping at the free margin and covered by a ciliated 

 epithelium. The interlaminar space, so unquestionable in the 

 gill-leaf o{ Patella (fig. 6 e), is here however much more contracted 



