36 Dr. T. Williams on the Mechanism of Aqua tic 



parallel non-cornmunicating tubes, of regular outline and uniform 

 diameter in the Lamellibranchiata, of irregular contour and vari- 

 able diameter in the Cephalophora. In all, the ultimate blood- 

 channel constitutes one, single, independent tube from one end of 

 its course to the other. Returning upon itself it does not lose its 

 individuality : it nowhere inosculates : it reticulates in no single 

 instance. One foundational law of structure is thus proved to 

 preside over the disposition of the ultimate elements of the 

 branchial orgaus in all the mollusks above the Tunicata. Already 

 the thoughtful eye descries the bright, continuous thread of 

 ' principle ' linking remotely separated and disjointed varieties 

 into the golden chain of consistent unity. Another generality no 

 less remarkable remains to be propounded. The branchiae of all 

 Tunicate and all Lamellibranchiate, and a considerable majority 

 of the Gasteropod mollusks are penetrated by the aerating water. 

 The branchia is a sieve through which the water filters. This 

 act of branchial nitration is a fundamental fact in the history of 

 all the inferior mollusks. The area which is circumscribed by 

 the mantle, at least in all Tunicate and Lamellibranchiate mol- 

 lusks, is divided more or less completely into two distinct com- 

 partments, the one pallial and external, the other internal and 

 visceral (PI. I. fig. 7, c & d; PI. II. figs. 9 & 13). The bran- 

 chiae constitute cribriform plicae developed on the divisional 

 membrane (fig. 7, e, e, e, e) by which these two compartments (c,d) 

 are separated. 



These leading propositions outlined in brief, will suffice to 

 prepare the mind for the right conception of those interesting 

 details which it is now proposed to consider. 



The limits of these papers render it impossible to refer in ex- 

 tenso to those anatomic specialties by which the branchial organs 

 of every species are more or less differentially characterized. 

 Those only can be selected for study which involve a typical 

 principle. Rules, not exceptions, it must be the aim of these 

 investigations to define. 



Tunicata. 



Tunicate mollusks stand immediately above the Bryozoon. 

 From the latter they are distinguished in the possession of a 

 heart. The movement of the blood is due exclusively to the 

 contractions of this central organ. The heart is systemic and 

 tubular. In many genera it is valveless, as indicated by alterna- 

 tions of direction in the blood's course. No definitely parieted 

 vessels occur on any segment of the peripheric arc of the circu- 

 latory system. To this rule the branchial forms an exception. 

 The branchial " bars " are, however, not ordinary vessels. They 

 are peculiarly formed. They are not analogous to those of the 



