82 Dr. T. Williams on the Mechanism of Aquatic 



parts in all others. That border which lodges the cartilage may 

 be called the dorsal or cartilaginous (fig. 4 a-f) ; that which 

 stretches from the distal point of the cartilage to the extreme 

 end of the base, the free or flexible border {a, h, g) formed of the 

 vascular loops ; and thirdly, that may be called the fixed side 

 {c, c) which is attached to the vault of the cavity throughout its 

 entire extent, and is the mathematical base of the triangle. In 

 some genera a portion of the gill is described as pendent and 

 floating in the cavity. The power to exsert the gill is enjoyed 

 by Valvata. It is, however, so exceptional a character, that the 

 branchia in nearly every Pectinibranch may be stated to be 

 sessile on a fixed base. But in describing the gills of this order, 

 systematic naturalists without exception commit another error. 

 In the Siphonostomata, embracing the carnivorous Gasteropods, 

 the '' branchial plumes are stated to be double, or to be two in 

 number." In the Holostomata they are said on the contrary to 

 be ^' single," that is, that there is but one branchial plume. If 

 this error did not originate with, it certainly has been perpetuated 

 by Dr. Sharpey. In his article " Cilia," in the ' Cyclopaedia of 

 Anatomy and Physiology,' he states that in reflecting the roof 

 of the branchial chamber in Buccinum, two sets of gills are seen, 

 one of which consists of two rows of laminae (fig. 2 b), the other 

 of one row («, a). That structure which Dr. Sharpey describes 

 as a " gill with two rows of laminae," which is attached to the 

 extreme left of the vault, is a gland (fig. 2 b ; fig. 1 d). The de- 

 tails of this point will be given on another occasion. In external 

 characters it looks like a gill. No one but the microscopic 

 anatomist could note a difference. A deep difference however 

 does exist. Here again is exemplified the service which minute 

 special anatomy may render to the cause of general physi- 

 ology. 



His researches enable the author to affirm with confidence, 

 that in all the Pectinibranchiata the gill is a single organ. Though 

 in some of the Cyclobranchiata the organ is double, and may 

 exhibit a bilateral symmetry, in the Pectinibranchs it is single. 

 To this rule there is no exception. Since the constituent parts 

 of every pectinibranchiate gill consist of triangular or fan- 

 shaped leaves, strengthened at the dorsal border by a comb's 

 tooth-like process of cartilage, the terms for the construction of 

 an accurate and consistent general definition of the branchiae in 

 this family of Mollusks are established. A subdivision of these 

 organs into two leading classes becomes essential, however, with 

 a view to a more accurate description of structural minutiae. In 

 the genera Buccinum and Littorina the extreme representatives 

 of these two classes occur. The branchial leaf of the former 

 (fig. 4) is distinguished by smooth sides — that is, it is an unpli- 



