282 



MANUAL OF ZOOLOGY 



mo 



ulating together and partly overlapping one another. They are some- 

 times partly, sometimes completely, covered over by the mantle. Each 

 valve consists of two very distinct layers, a more superficial and a deeper, 

 the latter formed of a compact calcareous substance, the former perfo- 

 rated by numerous vertical canals for the lodgment of the sense organs, 

 to be presently referred to. External to the valves the dorsal integu- 

 ment (mantle) of Chiton and its allies is usually beset with a number 

 of horny or calcified tubercles and spicules. The mantle develops only 

 very slight lateral flaps, and under cover of these are a series of small 

 gills or ctenidia (Fig. 169, ctn), to the 

 number of fourteen to eighty. The 

 mouth and anus are both median, situ- 

 ated at the anterior and posterior 

 ^ TWTi^^mm 1 V extremities respectively. 



The buccal cavity always contains a 

 well-developed odontophore. The in- 

 testine is elongated and coiled. There 

 are salivary glands and a large paired 

 --ft liver. There is a well-developed heart, 

 consisting of a median ventricle and 

 two lateral auricles. The pericardial 

 cavity in which it lies is a space of con- 

 siderable extent in the posterior region 

 of the body, below the two last valves 

 of the shell. 



The central part of the nervous sys- 

 tem comprises an oesophageal nerve- 

 ring, consisting of a thicker dorsal 

 cerebral portion not differentiated into 

 ganglia, and a thinner ventral buccal commissure. Two pairs of longi- 

 tudinal nerve-cords, pedal and pallial, are given off from this poste- 

 riorly. The former, which give off nerves to the foot, are joined by 

 numerous commissures passing beneath the enteric canal. The large 

 cords contain nerve-cells throughout their length. 



The conspicuous organs of special sense present on the head of Gas- 

 tropods (see p. 289) are absent in the Chitons. A pair of processes 

 situated in front at the sides of the mouth have the character of labial 

 palps. Remarkable sensory organs, the micrastketes and the megalcES- 

 thetes, lie in the canals already mentioned as occurring in the super- 



maril 



clen 



CCTL 



Fig. 169. — Chiton, ventral view. 

 an, anus; cten, ctenidia; ft, foot; 

 mant, mantle edge; mo, mouth. 

 (After Pelseneer.) 



