Quarterly Journal of Couchology. 403 



Pectinidce, Dreisscna polyuwrpha, &c., these latter being of a fibrous 

 nature, and the product of a special organ. 



As members of the order Pulmonohrandiiata — breathers of 

 atmospheric air — spin and use threads oftener than any other of 

 the Gasteropoda, especially the aquatic members of the group, and 

 as their method of using them differs from the FcctinibrancJiiata — 

 water-breathers — we will consider them first. 



In order to be better understood, let me describe briefly 

 their process of respiration. On the side of the creature is 

 situated a sac, or branchial chamber, formed by a fold in the 

 mantle, and having an opening outwards, which the animal can 

 open and shut at will. The air in this sack is renewed by diffusion 

 while the moUusk is at the surface of the water, which air 

 oxygenates the blood through the veins, which ramify in an 

 arborescent form over the roof of the cavity. Now it will be 

 obvious to the reader that when this sac is distended with air, 

 the creature becomes of less specific gravity than water; hence it 

 will float, even against its own will, when dislodged from its hold; 

 and, on the other hand, when the air in its branchial chamber is 

 exhausted by natural respiration, or expelled by reason of some 

 annoyance, the creature, becoming heavier than water, at once 

 sinks to the bottom; and on this simple fact hangs the capability 

 of the moUusk to spin an upiuard or doiomvard thread. 



I have never seen a member of this order descend by a thread 

 unless it had first ascended by one, in Vv^hich case it might return 

 upon the same thread. It would no doubt be possible for it to 

 descend by a thread if its air-chamber was sufficiently empty to 

 allow of it sinking; but, atmospheric air being essential to the 

 creature's existence, it very rarely voluntarily descends without a 

 supply, and never in such a case by a thread, although it will creep 

 about in the water when the air in its branchial cavity is 

 sufficiently exhausted to allow it to fall to the bottom of the water 

 when loosed from its hold. 



