52 COMPARATIVE ANATOMY chap. 



withdrawn into the shell. Siphons united throughout their whole length are found 

 in tlie 3f((cirida; a few Veneracea, Ltitraria, Solenocurtus, Solen, the Pliolddidcv, 

 many Anatiiiidce, and the C'lavagellidcc. 



In some cases, siphons which are united for some distance at the base, separate 

 near their ends and even diverge, e.g. in Pctricola among the Vcnci'acca, Teredo, etc. 



The two siphons are often of unequal length. In the Modiolaria {Mytilida') only 

 one, the anal, is developed, while the branchial aperture remains unseparated from 

 the large mantle cleft. The reverse is the case in Drcissensia and Scrohlcularw, 

 where the branchial siphon is much longer than the anal. 



The siphons are sometimes provided with valves ; these occur more often in the 

 anal than in the branchial siphon. 



Significance of the development of the Anal and Branchial Apertures 

 and Siphons. 



Most Lamcllibranchia inhabit mud ur sand, into which they sink the anterior part 

 of the body, burrowing by means of the protrusible foot. The water necessary for 

 bathing the gills and for respiration can only be received into and expelled from the 

 mantle cavity through the cleft at the posterior end of the body which projects above 

 the mud. The fcecal masses from the anus near this point must also here be ejected 

 from the cavity. The development of localised inhalent and exhalent apertures is 

 explained by the fact that a constant regulated stream of water into and out of the 

 mantle cavity is necessary both for respiration and for conducting particles of food 



Fio. 5S.— Scrobicularia piperata buried in mud. The inhalent siphon tal;es in niuil as 

 nourishiaent ; tlic anal siphon stands erect (after Meyer and Mobius). 



to the mouth. The most advantageous point for the exhalent aperture is obviously 

 directly behind the anus. 



Siphons attain development in consequence of the habit of life of many bivalves, 

 which bury themselves deep in mud, sand, wood, and even rock. By means of theii- 

 siphons they can still remain connected with the water which bathes the surface of 

 their place of concealment, and, as long as the animals remain undistin-bed, a con- 

 stant current enters the mantle cavity through the branchial and leaves it through 

 the anal siphon. 



Where the mantle folds have grown together to a large extent (closed mantle) the 

 siphons are always well developed. Such closing of the mantle is found principally 

 among bivalves which bore into wood, clay, rock, etc., and in which the foot of the 

 adult is weakly developed, or altogether rudimentary. The degeneration of the foot 

 leads to the shortening of the pedal aperture which originally served for its pro- 

 trusion. 



The mantle is found comj^letely open with only slightly developed anal and 

 branchial apertures or none at all, in bivalves which do not burrow, l)ut live 

 surrounded by water, either attached to the bottom or lying freely on it. 



