THE ANIMAL, ETC., OF CHITON. 113 



PLATE XV.. 



The Animal, Shell-plates, etc., 

 of Chiton. 



The Family of the Chitonidae constitute a Sub- 

 class of the GASTROPODA in the PHYLUM 



Mollusca. 

 The Sub-class is ISOPLEUKA (PLACOPHOKA). 



" The Chitons, or Coat-of-Mail Shell-fish, differ from 

 all other molluscs in that the shell is formed of eight 

 pieces overlapping. In the larval condition, the young 

 are covered with calcareous spicules before the formation 

 of the plates. The animals are sluggish in movement, and 

 frequent rocks at low tide." (P. J. E. See descriptive 

 label in Plate opposite.) 



" The shell is composed of eight transverse 

 imbricating plates, lodged in a coriaceous mantle, which 

 forms an expanded margin round the body. The first 

 seven plates have posterior apices ; the eighth has its apex 

 nearly in front. The six middle plates are each divided 

 by lines of sculpturing into a dorsal, and two lateral areas. 

 All are inserted in^o the mantle of the animal by processes 



(apophyses) from their front margins The 



border of the mantle is either bare, or covered with minute 

 plates, hairs, or spines." (S. P. Woodward, Manual of the 

 Mollusca, p. 156). 



" The head is without tentacles and sense-organs. 

 The foot occupies the greater part of the ventral surface, 

 and has a broad, flat sole. The mantle occupies the whole 

 dorsal surface, and completely hides the head. The 

 visceral sac is flattened and not drawn out into a dome. 

 The mantle-fold encloses a shallow groove which 

 completely surrounds the body, and is roughly of uniform 

 depth throughout. The ctenidia, which are bipectinate 

 and projecting, are attached to the floor of this groove, 

 generally in the hinder part, but in some forms they 

 extend forwards nearly to the head ; they vary in number 

 from six to eighty pairs. The sexes are separate. There 

 is only one family, but there is considerable variation in 

 the group, both specific and individual, and consequently, 

 a large number of species has been described." 

 (A. Sedgwick. A Student's Text-Book of Zoology, pp. 387-392). 



