42 AMERICAN JOURNAL 



Type MOLLUSCA, Cuvier (emend.) 

 Heterogangliata, Owen, 1835. Saecata, Morse and Hyatt, 1865. 



Animals characterized by the development of organs singly 

 or in single pairs,* with the inarticulated, non-radiated soft parts 

 enveloped in a sacf or mantle of varied form, which secretes the 

 usually external hard parts, if any exist, and which is invariably 

 pierced for the oral aperture, and generally more or less open 

 elsewhere ; furnished with a mouth and intestine which is bent 

 more or less upon itself; with a nervous ganglion below the oral 

 opening and usually nervous cords therefrom, forming a ring 

 about the oesophagus ; other ganglia, if any, distributed in a 

 scattered manner and never along a pair of symmetrical, sub- 

 abdominal, median cords |. Animals free, or having an organic 

 connection with extraneous objects. 



The respiratory, reproductive and circulatory systems are so 

 variable and offer such wide discrepancies, that no characters are 

 afforded by them for the typical diagnosis. 



The same may be said of their embryology. The type, or sub- 

 kingdom, offers two sub-types or divisions, but we have not the 

 data to exactly define the limits of the gap between them. The 

 following diagnoses are generally applicable. 



Sub-type Mollusca vera. 



Mollusca having three principal ganglia (or pairs of ganglia); 

 a heart and a circulation within definite vessels ; the heart sit- 

 uated on the opposite side of the intestinal canal from the sub- 

 oesophageal ganglion or encircling the intestine, or rarely double 

 or possessing accessory "pulsatile vesicles," (as in the cephalo- 

 pods) in connection with the branchial arteries ; reproducing by 

 ova only ; breathing free air or water ; usually possessing a 

 "water-vascular" system. 



This subdivision presents three well marked classes, the Ce- 

 phalopoda, Gasteropoda and Conehifera, which are, however, of 

 very unequal value. The last is much more widely separated 

 from either of the others than they are from each other, and, as 



* Clark, Mind in Nature, p. 195, 1865. 



f Morse, Class. Moll, on the principle of Cephalization, Com. Essex 

 Inst. p. 178, 1865. 



% Owen, Enc. Brit. Art. Mollusca ; Ed. viii, p. 320, vol. xv, 1858. The 

 nerves of the appendix in Appendicularia are furnished with successive 

 dilations, but they are not sub-abdominal nor bilaterally symmetrical, 

 being on each side irregularly distributed without reference to those on 

 the opposite side. 



