^JHWi Dr. T. Williams on the Mechanism of Aquatic 



of this series*. The primary ganglia always exist ; many of the 

 local parieto-splancluiic may be absent. The latter in size bear 

 relation to the organ or part to whieh they furnish nerves. 

 iThose connected with the branchiae vary with the latter organs. 

 In the Mollusca, therefore, a part of the peripheric structures 

 may acquire increased development, while the central systems 

 remain unchangecj.jg gg^ ^^^^ ^^ ^.^^ sidT .sebilbJB^ odi lo 



bisifPh^sics of the Respiratory Chamber in the Cephalophor^ odJ 



In the Cephalophora the organs of respiration assume variable 

 positions as regards the rest of the body. They lack the topo- 

 graphical constancy of these organs in the Lamellibranchs. As 

 in the latter class, in the Cephalophora they consist essentially 

 of developments of the tegumentary system. They are elaborated 

 productions of the mantle. Although they may vary in struc- 

 ture and position, this relationship to the mantle is never radi- 

 cally affected. A brief review of the space or chamber in which 

 the branchiae are enclosed, will enable us to enter more detailedly 

 into the regional anatomy of these organs. 



The respiratory chamber (fig. 1 /) in the Cyclobranchiata is 

 but imperfectly defined. It is for the most part a grooved cir- 

 cular fossa between the edge of the foot and the border of the 

 mantle (fig. 1 h) ; but the branchiae are not the less a develop- 

 ment of this structure. If in the Patelloid and Chitonoid forms 

 the edge of the mantle were prolonged and introverted, a channel 

 would be defined in which the branchiae would be enclosed. 

 Wherever these organs are placed, some provision, such as a 

 groove, is made for the efficient play of the physics of the bran- 

 chial process. 



The direction of the main aerating currents is from before 

 backwards, and transversely on the branchial leaflets. The 

 cavity which circumscribes the branchiae in the Fissurellidae com- 

 mences at the neck and extends some distance backwards along 

 the sides. It receives water though the vertical fissure, placed 

 between the pedicles of the branchiae ; it escapes expiratorily at 

 the lower and posterior border. In Emarginula the respiratoiy 

 cavity of the mantle is situated at the back of the neck. In the 

 Haliotidae this cavity is similarly formed. Patella, Acmcea, 

 Pileopsis and Calyptraa are the cervico-branchiate patelloid 

 forms with a single non-symmetrical branchia. Fissurella, 

 Emarginuhj Puncturella and Haliotis are in the same cervico- 

 branchiate category, but having two branchial leaves. [Clark.) 



' .Thus in the same family how striking are the generic varia- 



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