. A manual of zoology. 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 mant
. A manual of zoology. 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, cln), to the number of fourteen to eighty. The mouth and anus are both median, situ- ated at the anterior and posterior 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 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 micrces
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