Xll POLYPLACOPHORA. 



pores or tufts has been considered by systematists a character of 

 great value ; but not with justice. The fact is that at least three 

 times in the development of Chitons has this feature appeared 

 independently ; first in the genus Hemiarthrum of Lepidopleuridce, 

 this form having a pore at each suture, and six around the head 

 valve ; second in Callistoplatince (culminating in Mopaliidaf) where 

 the pores are excessively inconstant, and the number around the 

 head valve indefinite (0-6) ; and for the third time in Acantho' 

 chitidce and its derivative Cryptoplacidce, where the number of 

 tufts around the head valve is invariably four, in genera having 

 pores. Every genus (if it has more than one or two species) con- 

 taining pore-bearing species, includes also forms in which the pores 

 or tufts have become subobsolete or entirely lost. In the genera of 

 Mopaliidce, the pores are occasionally not of specific importance ; 

 and the same is true of Amicula. 



The genesis of sutural tufts is not beyond the reach of theory. I 

 believe them to be the direct result of over-nutrition of the girdle 

 at the sutures, caused by its greater mobility at those points. It is 

 obvious that every motion of the valves must cause a flexure of the 

 girdle at the sutures; this brings a greater flow of nutriment to the 

 stimulated portion, in this case a narrow strip, resulting in a more 

 exuberant growth there. 



HEAD, FOOT, GILLS, ETC. 



Viewed ventrally Chiton presents a fleshy foot, similar to that 

 of the Gastropoda (pi. 34, fig. 68). Separated from this by a sinus is 

 the head, with the mouth in the center of the disk, which is gen- 

 erally surrounded in front by a lappet called by Carpenter the 

 " veil. " There are no tentacles or eyes. The inner edge of the 

 ventral surface of the girdle may be regarded as the mantle. It is 

 sometimes fringed or produced into finger-shaped processes in front. 

 The gills consist of a row of triangular branchial plumes extending 

 from near the tail forward from a fourth to the entire length of the 

 foot. The anus is median and posterior, generally opening on 

 a papilla. The two nephridial openings are in the gill-groove, at the 

 side and in front of the anus. The two genital openings are in 

 front of the nephridial orifices. 



The radula has been investigated by Lpven, Sars, and especially 

 by Dall, who describes the teeth as follows : " In all Chitons exam- 

 ined there is a simply cuspid rhachidian tooth, and on each side a 



