CHITONID^. 243 



surella can scarcely be withdrawn within the shell, studded 

 alike in both with papillee and rugosities, and the same mar- 

 ginal fringes. The nervous masses in the two accord closely: 

 the only exception is the striking, but really unimportant, 

 division of the cone of the Chitons into segments; but this 

 incident may be accounted for on very simple grounds. We 

 consider the fracture of the shell not a character denoting an 

 affinity with the Annelida, as the body of the animal has no 

 corresponding articulations, but simply an aid to facilitate 

 locomotion. The foot of the strict symmetrical testaceous 

 Gasteropoda is generally nearly concurrent with the length 

 of the body, and forms its base, from which, by a gradually 

 increasing cone, the animal becomes at maturity fixed to the 

 summit of the shell by a powerful muscle : this structure 

 united to an entire cone, and combined, as in the Bivalves, 

 with the depressing effects of a complete hermapliroditism, 

 that of Venus sine concubitu, almost extinguishes the loco- 

 motive functions ; and we see throughout the Patelloid tribe, 

 all of which have a foot co-extensive with the body, an almost 

 entire fixation to the same spot ; indeed we believe that some 

 of the PateUce pass their existence in the depressions of the 

 rock on which they are cast as embryos, and the same apathy 

 exists in Pileopsis and Calyptrcsa. But nature has judged fit 

 to give the Chitons additional motive powers, for when fixed 

 on the area of a smooth pebble, they will travel off it, whilst 

 the Patella are immoveable : this is owing to the integrity of 

 the cone, and the enormous muscle which fixes the animal to 

 its apex. This structure does not permit the body sufficient 

 flexibility for much progression, and almost confines it to a 

 vertical elevation and depression; but the Chitons, by the 

 segmental condition of their shells, have accorded to them 

 sufficient flexibility to obtain a vermicular motion, and its 

 consequence, a greater facility of march. The Trachelipoda 

 have infinitely superior attributes for motion, from the foot 

 being fixed to a small portion of the body by an elastic 

 cylindrical pedicle that affords a perfect pliability, and the 

 acquisition of the necessary undulatory quality to effect a 

 comparatively facile progression. 



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