MOLLUSCOUS ANIMALS. 395 



remarked that this latter arrangement does not materially alter the na- 

 ture of the processes performed, but is an adaptation of the common struc- 

 ture to the cases of living within sand or mud, or at some depth in it, 

 the length of the siphons being adapted to the habit of the creature in 

 this respect. It is nevertheless to be admitted that the simpler struc- 

 ture, where the water is directed in its proper course, though the mantle 

 lobes and branchial lamellae are entirely unconnected, is lower than the 

 more complicated one, in which there is more or less coherence of these 

 parts, and extension where required into elongated siphons : accordingly 

 the differences in this respect may be assumed as valuable aids in classifi- 

 cation, though here, as with all other characters, we meet with excep- 

 tions which might greatly embarrass us if we did not recognise them as 

 deviative cases, necessarily belonging to a truly natural system. Who 

 would remove Dreissena from Mytilidtv. on account of its siphons ? I 

 have referred to the varying length of the siphons, and this has given 

 occasion to another sectional distinction, much, and justly, valued since 

 it has come into notice : I mean the distinction of all Lamellibranchiata 

 into Integropallialia and Sinupallialia. The margin of the mantle-lobes 

 leaves its impression on the shell. If there are no siphons, or short ones, 

 the retraction of which within the shell does not disturb the position of 

 the mantle lobes, the pallial impression will follow the curve of the mar- 

 gin of the shell all round, and is said to be entire, thus characterising the 

 Integropallialia. If, on the other hand, the siphons are so much elon- 

 gated that the muscular effort in their retraction draws back also a part 

 of the mantle lobes, then the frequent repetition of this action will be 

 marked in the pallial impression by a sinus, more or less deep, immediately 

 behind the position of the siphons, in which case the shells are Sinupal- 

 lialia. As we have hgre an index, in the appearance of the shell, to an 

 important structural difference in the animals, the character is one of 

 great utility. It would not, alone, distinguish orders, since it would not 

 distinguish Asiphonida from Siphonida with short tubes, and a recooni- 

 tion of the importance of the length of the Siphons, implies that of their 

 presence or absence, but combined with other characters, its great utility 

 ought to be admitted. The form of the foot is undoubtedly a character 

 of high importance, both because the varied development of this charac- 

 teristic molluscan organ, might naturally be expected to yield valuable 

 results, and because we caq see the connection between its different con- 

 ditions and different habits of life requiring it to be differently em- 

 ployed, as for the attachment of byssus, crawling, jumping, or boring, 



