54 Dr. T. Williams on the Mechanism of Aquatic 



The character of this exhalent current is not the same in all 

 genera. In Cardium it is much more regular than in Pholas, 

 &c. It occurs quite as distinctly in the Asiphonophora as in 

 the Siphonal classes. It is with this current that the rejecta- 

 menta escape. 



The evidence upon which the doctrine of the independence of 

 this cavity from the ventral is maintained, will be adduced when 

 speaking of the minute, structure of the branchiae. 



If the view supported by Mr. Hancock be true, that the 

 dorsal or intra- branchial cavity is separated from the ventral or 

 extra-branchial by a partitional membrane which is permeable 

 nowhere but at the branchial stigmata, it follows that every- 

 thing which passes from one chamber into the other, either in a 

 progressive or regressive direction, must percolate through these 

 minute foramina. The author's dissections, however, render it 

 probable that a fissured opening (fig. 6, e) at either base of the 

 foot exists in some of the siphonal genera, if not in others. It is 

 seated at the base of the branchial lamella? and the junction of 

 the vertical partition of the mantle with each superior gill, and 

 opens directly from the intra- into the e,r/ra-branchial chambers. 

 The office of this fissure is that of a safety-valve. 



When the outlet of the ex-current siphon is closed by sphinc- 

 teric contraction, the intra-branchial cavity being rapidly more 

 and more filled in virtue of the continuousness of the branchial 

 ciliary action, the surplus fluid escapes through the lateral 

 fissures back again into the ventral or extra-branchial chamber, 

 again to pass through the branchial foramina, propelled by cilia. 

 It is by thus repeatedly filtering the same water through the 

 branchiae that those bivalves, such as the Mussels, sustain life, 

 though abstracted for a considerable time from their native 

 element. The fissure in question is detectable only from the 

 inside of either chamber, not from the outside view of the whole 

 mantle, even after the separation of the animal by spirits of 

 wine from its shell. If the exit of the water from this intra- 

 branchial enclosure be due to the force exerted by the lamellar 

 cilia, it follows that the egressing current should be equally as 

 vigorous and marked in the non-siphonal as in the siphonal 

 genera. The presence of the ex-current siphon does not affect 

 the real branchial action of respiration. As a tubular extension 

 of the cavity, it enables the contents of the latter to be delivered 

 at a greater distance away from the body. This is its real office. 

 Thus the sphere in which the animal lives is maintained in 

 purity. The in-current which occasionally occurs through this 

 dorsal or intra-branchial siphon should be regarded really as a 

 momentary accident, as an irregular reversal of a normal current. 

 Water thus drawn into the cavity of the ex-currcnt siphon can 



