30 Dr. T. Williams on the Mechanism of Aquatic 



is not exclusively a respiratory chamber. It serves to lodge 

 other organs. It contains the hearty the termination of the in- 

 testine, the excretory ducts of the repi'oductive system, and 

 several varieties of glands (PI. V. figs. 1 & 2). It is therefore 

 by no means an unimportant part of the body of the moUusk. 

 Between the position of the rectum and that of the branchiae 

 there obtains in this, as in the Pulmonifera, a constant relation- 

 ship : one reason for this relationship is a mechanical necessity. 

 The egressiug current from the gills is thus adapted collaterally 

 to convey away from the body the faecal excreta. This current 

 connects itself also with the functions of the generative system. 

 It forms a vehicle for the mucus supplied by the glands of this 

 chamber. By its aid the latter is enabled to invest the ova as 

 they escape from the body — constituting thus a cocoon in which 

 they are temporarily cradled. It is by skilfully subordinating 

 the office of one organ to that of another, that Nature^s contri- 

 vance surpasses man's. In her machinery a force is nowhere 

 allowed to transpire unutilized. It is always deflected to a pur- 

 pose ; though sometimes to one whose significance ma}' prove 

 illegible to her observers. 



As the details to which the reader's attention is now solicited 

 are novel, and now for the first time published, it is desirable 

 that a clear and concise method of presenting them should be 

 adopted. The author proposes in the first place to describe the 

 branchiae in those genera especially of this order in which he has 

 subjected these organs to a special examination. He v.ill then 

 return to a consideration of the glandular apparatus of this im- 

 portant cavity, and finally deduce such inferences with respect 

 to their purpose and function, as their chemistry and the facts 

 of their minute structure may appear to warrant. 



The branchiae of the Pectinibranchs (PI. V. figs. ] & 2 c' f/') are 

 almost always fixed to the roof of the cavity in which they are 

 lodged. They occupy an intermediate position between the 

 " gland of the mucosity" (fig. 1 b) and the colour-gland (fig. 1 a, 

 fig. 2 h), which lies invariably to the extreme left. In some 

 genera the gill is placed at the extreme left of the chamber — at 

 a point, that is, which is the direct opposite of that of the rec- 

 tum ([ig. 1 c). This position is significant of the care with 

 which Nature locates the breathing organ in order that it may 

 receive the most direct influence of the aerating current as the 

 latter enters the cavity. In the following account few difl"ereuces 

 in the relative anatomy of this organ will demand attention. 

 The most striking diversities will be found to aftect the figure or 

 outline-form and ultimate structure of the individual laminae or 

 pectinations of the gill. These objects are entirely and exclu- 

 sively microscopic (PI. V. figs. 3, 4, 9, 13, 14) : they are re- 



