CHITONS. 



399 



DIFFERENT STAGES OF THE LARVA OF CHITON (magnified). 



characteristic of the different groups into which the Chitonidce have been divided. 

 The gills are placed on each side between the mantle and the foot, and vary in 

 length. Adult chitons have neither eyes in their head nor tentacles, but the 

 mouth is provided with a long radula, the teeth of which are arranged in a very 

 peculiar complicated manner. The chitons are bisexual, but, like the limpets, 

 destitute of certain functional organs. In the early stages the young chiton bears 

 no resemblance whatever to its 

 parent. This can be appreciated by 

 observing the accompanying figures 

 illustrating three stages in the 

 development of Chiton marginatus, 

 one of the dozen species found on 

 the British coasts. The embryo (A) 

 is a spherical body, £j of an inch in 

 diameter, divided into two unequal 

 parts, the dividing line being marked 

 by a row of cilia, and a tuft of similar cilia being situated at the vertex of the 

 smaller half. The eyes are visible upon the lower portion below the cilia. In a 

 later stage (B and 0) the division of the back into eight sections is remarkable 

 and unique in the Mollusca. At this stage the foot begins to develop, the forepart 

 of the animal being ciliated. In later stages of the development the eyes and cilia 

 disappear, the forepart shrinks up into the ridge surrounding the mouth, and the 

 back develops the eight shelly plates. It has already been stated that the adult 

 Chiton has no eyes. This is only true as regards the head of the mollusc, for 

 Moseley made the discovery that certain forms have the shell studded with eyes, of 



which as many as eleven to twelve thou- 

 sand sometimes exist in a single individual. 

 They are not unlike those already referred 

 to as present on the back of Onchidium. 

 Chitons live principally on rocks and 

 under stones at low water, or at moderate 

 depths, but a few have been obtained as 

 low down as two thousand three hundred 

 fathoms. They are all marine, very slug- 

 gish in their movements, and, if disturbed 

 from their resting-place, roll themselves 

 up into a ball like a wood-louse. The 

 number of recent species is considerable, 

 and they appear to range all over the globe. Fossil remains of certain forms have 

 been found in most geological periods, since the Silurian. 



" V "Nv 



eyed chiton (ScMzocMton incisus). Anterior valve, 

 with six rows of eyes (enlarged). 



Order Aplacophora. 



The molluscs of this order are somewhat worm-like, with a mantle enclosing 

 the entire body, but not secreting a shell, and more or less studded with spicules. 

 They are related to the chitons by certain points in their anatomy, especially with 



