MOLLUSCS. 289 



every other respect agree closely with each other being occasionally widely separated. 

 The characters derived should be compared with those obtained from other structures 

 and thus all such extremes would be avoided. 



In most forms the body is distorted to fit 

 the spiral shell, and even where this is not 

 the case, the alimentary canal usually follows 

 a tortuous coui'se, doubling on itself and 

 terminating usually on the right side of the 

 body, frequently in front of the middle. The 

 cavity of the mouth communicates with an 

 a'sophagus which sometimes dilates into a 



soi't of crop, and eventually empties into the stomach, from which arises the intestine 

 either opposite to or beside the cardiac opening. In a few forms the stomach is armed 

 with plates or horny teeth. Salivary glands are almost always present, usually two in 

 numljer, but occasionally four are found, and it is interesting to note, in passing, that in 

 Uolium and some other forms tliese glands pour out a saliva containing sulphuric acid. 

 The liver is well develo[ied. 



The circulatory system is usually well developed, though in DentaUum a heart is 

 wanting, while other forms show a correspondence to the Acephals. Thus in the ear 

 and top shells, the alimentary canal perforates the heart, while in the first, as in the 

 chitons, there are two auricles. Usually, however, there is one auricle and one ven- 

 tricle, which propel a blood containing colorless, nucleated corpuscles. As in the 

 Acephals, the heart receives the blood from the gills and forces it to all parts of the 

 body. 



Respiration is effected by means of gills or by pulmonary organs. The gills, which 

 are usually contained in a cavity of the mantle, may consist either of lamellar organs 

 or of plume-like branchiifi. Lankester, who has recently investigated the structure of 

 the gills, is of the opinion that the primitive type was what he calls a ctenidium, con- 

 sisting of a central stalk to which lamellar respiratory processes were attached, a view 

 which seems open to some objections. The variations occurring in the respiratory 

 organs are of great value in systematic work, and will be referred to again in connec- 

 tion with the different grou])S. The pulmonary cavity of the Pulmonifera is formed 

 by a cavity of the mantle, and is richly supplied with blood vessels which extract the 

 oxygen necessary for respiration from the air. In a few forms special res])iratory 

 organs are lacking. 



Closely connected with the organs of circulation arc the i-enal organs. There are 

 one or more sacs !iear the heart and opening to the exterior, whic^h extract their secre- 

 tion from the blood going to the heart, and convey it outside the body. 



The nervous system acquires a different development in the various groups. In 

 the tooth shells it most nearly approaches that of the lamellibranchs, consisting of 

 two ganglia above the oesophagus, connected by two nervous cords with a pair of 

 pedal ganglia, while two more cords connect the brain with two ganglia near the vent. 

 These two last are evidently comparable to the visceral ganglia of the bivalve mol- 

 luscs. In the other forms the visceral ganglia may be increased in size and number, 

 and become closely connected with the other two i)airs. The details of structure 

 should be sought in more technical works. 



The auditory organs are usually seated near the ganglia of the foot. Kyes also 

 are generally present, and usually are two in number, situati'd upon tiie head or its 



VOL. I. —19 



