174 MORSE, CLASSIFICATION OF MOLLUSCA 



While we have thus seen that the area of attachment 

 first springs from the dorsal region, and gradually changes 

 as we ascend in structure toward the anterior end, so we 

 find the principal organ of locomotion, i. e. the foot, is 

 first developed from the ventral region, and in like manner 

 tending toward the anterior end, as we ascend in the scale, 

 until, in Cephalopoda, the specialized divisions of the foot 

 surround the head, and point directly forward. 



Having personally communicated the substance of this 

 paper to Professor James D. Dana, he has, in a letter to me, 

 indicated certain gradient relations among the Lamelli- 

 branchs, Gasteropods, and Cephalopods, as manifested in 

 the special characteristics of the head, or anterior part of 

 the body, so clearly illustrating the principle of Cephaliza- 

 tion that I now take the liberty of presenting them. In 

 the Lamellibranchs the foot is a simple muscular organ 

 developed from the ventral surface and protruding anteri- 

 orly. It is simply an organ of locomotion, in the lower 

 forms not even performing this function. The oral open- 

 ing is a simple slit, without the power of seizing or tritura- 

 ting its food. 



In the Gasteropods the foot is more specialized, and as 

 an organ of locomotion far superior to that of the Lamelli- 

 branchiates, having oftentimes three well characterized re- 

 gions, called by Huxley, the pro- meso- and metapodium, 

 these regions oftentimes supporting certain processes, 

 e. g. cirri, opercula. The foot not only performs locomo- 

 tion but in many cases has the power of seizing and re- 

 taining its prey (e. g. Natica). The mouth has an appar- 

 atus for biting and triturating its food, being furnished 

 with an upper jaw, or buccal plate, and a tongue, armed 

 with silicious particles. In the Cephalopoda the foot is so 

 far differentiated as to be separated into prehensile arms 

 furnished with rows of suckers, or hooks. These arms sur- 

 round the head, and are thrown directly forward. They 

 are capable not only of locomotion, but of seizing their 

 prey, and performing also movements of aggressive action. 

 In the higher forms of Cephalopods, the function of loco- 

 motion is delegated to other organs, while the arms sub- 

 serve the uses of the head alone, and the mouth, furnished 

 with two powerful mandibles opposed vertically, forcibly 



