330 MARINE INVERTEBRATES 



weight, began to sag over to one side. Such a process would of 

 course tend to twist the digestive tract and bring the excretory 

 opening constantly farther forward on one side. At the same 

 time, this process would necessarily interfere, by the weight of 

 the overhanging viscera and the shell covering the mass, with the 

 development of the branchia? lying upon that side and now crowded 

 under the visceral mass. Thus, according to this theory, this 

 process has continued until, in the modern gasteropod, the diges- 

 tive tract has been bent upon itself and twisted from a straight 

 course into a curve of almost 180, bringing the excretory 

 opening near or just over the head, where it empties into the 

 mantle cavity ; while the original right gill, by the same move- 

 ment, has been brought to a position on the left side of the 

 head, forward of the heart, the original left gill having been 

 crowded out, eventually to atrophy and disappear. The same 

 torsion is found in the nerve-cords ; the heart, situated at about 

 the pivotal position of this twisting process, has turned about 

 upon itself, leaving an auricle in front of the ventricle, one 

 auricle, like one of the gills, having been lost. The mantle por- 

 tion covering the visceral hump naturally continued to secrete its 

 shell, though always in conformity with the change, the result 

 being the familiar spiral form of the usual gasteropod shell. 



This theory may not be satisfactory, but the asymmetry of 

 gasteropods is a problem to be solved, and a more interesting 

 line of biological investigation could not be found. 



Let us now take a good example of a gasteropod and locate its 

 various organs; at the same time we may use the occasion to 

 refer to more important modifications of these organs which will 

 be encountered later in the various genera. The most available 

 gasteropod on the east coast of the United States for this 

 purpose is Fulgur, both on account of its large size and its 

 abundance. Buccinum may be used if the student is north of 

 Cape Cod and therefore unable to secure a good living specimen 

 of Fulgur; the anatomical differences between the two are slight. 



Note the siphon protruding forward from a notch in the shell. 

 This consists merely in an elongation of a fold of the mantle, 

 which is held in a manner to constitute a tube, through which 



