PHYLUM MOLLUSCA 



281 



stance, the former perforated by numerous vertical canals for the lodgment 

 of the sense organs to be presently referred to. Externally to the valves 

 the dorsal integument (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 ctenidia (Fig. 157, cten.), to the number of from fourteen 

 to eighty. 



The buccal cavity always contains a well-developed odontophore. The 

 intestine is elongated and coiled. There are salivary glands and a large 



FIG. 156. Chiton SpinOSUS, dorsal view. (From the Cambridge Natural 

 History.) 



paired liver. The heart is well-developed, and consists of a median 

 ventricle and two lateral auricles. The pericardial cavity in which it lies 

 is a space of considerable extent in the posterior region of the body, 

 below the two last valves of the shell. 



The central part of the nervous system comprises an cesophageal 

 nerve-ring consisting of a thicker dorsal cerebral portion not differen- 

 tiated into ganglia, and a thinner ventral buccal commissure. Two 

 pairs of longitudinal nerve cords, pedal and pallia!, arise from this com- 

 missure posteriorly. 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. 



