CHITALDRUG, a district and town in the native state of Mysore, India. The district has an area of 4022 sq. m. and a population (1901) of 498,795. It is distinguished by its low rainfall and arid soil. It lies within the valley of the Vedavati or Hagari river, mostly dry in the hot season. Several parallel chains of hills, reaching an extreme height of 3800 ft., cross the district; otherwise it is a plain. The chief crops are cotton and flax; the chief manufactures are blankets and cotton cloth. The west of the district is served by the Southern Mahratta railway. The largest town in the district is Davangere (pop. 10,402). The town of CHITALDRUG, which is the district headquarters (pop. 1901, 5792), was formerly a military cantonment, but this was abandoned on account of its unhealthiness. It has massive fortifications erected under Hyder Ali and Tippoo Sahib towards the close of the 18th century; and near it on the west are remains of a city of the 2nd century A.D.
CHITON, the name[1] given to fairly common littoral animals of rather small size which belong to the phylum Mollusca, and, in the possession of a radula in the buccal cavity, resemble more especially the Gastropoda. Their most important characteristic in comparison with the latter is that they are, both in external and internal structure, bilaterally symmetrical. The dorsal integument or mantle bears, not a simple shell, but eight calcareous plates in longitudinal series articulating with each other. The ventral surface forms a flat creeping “foot,” and between mantle and foot is a pallial groove in which there is on each side a series of gills. Originally the Chitons were placed with the limpets, Patella, in Cuvier’s Cyclobranchia, an order of the Gastropoda. In 1876 H. von Jhering demonstrated the affinities of Neomenia and Chaetoderma, vermiform animals destitute of shell, with the Chitons, and placed them all in a division of worms which he named Amphineura. The discovery by A.A.W. Hubrecht in 1881 of a typical molluscan radula and odontophore in a new genus Proneomenia, allied to Neomenia, showed that the whole group belonged to the Mollusca. E. Ray Lankester (Ency. Brit., 9th ed., 1883) placed them under the name Isopleura as a subclass of Gastropoda. Paul Pelseneer (1906) raised the group to the rank of a class of Mollusca, under von Jhering’s name Amphineura.
The Amphineura are divided into two orders: (1) the Polyplacophora, or Chitons; (2) the Aplacophora, or forms without shells, Neomenia, Chaetoderma and their allies.
Order I.—Polyplacophora
| Fig. 1.—Three views of Chiton. |
| A. Dorsal view of Chiton Wosnessenksii, Midd., showing the eight shells. (After Middendorf.) B. View from the pedal surface of a species of Chiton from the Indian Ocean, p, foot; o, mouth (at the other end of the foot is seen the anus raised on a papilla); kr, oral fringe; br, the numerous ctenidia (branchial plumes); spreading beyond these, and all round the animal, is the mantle-skirt. (After Cuvier.) | C. The same species of Chiton, with the shells removed and the dorsal integument reflected, b, buccal mass; m, retractor muscles of the buccal mass; ov, ovary; od, oviduct; i, coils of intestines; ao, aorta; c’, left auricle; c, ventricle. |
| Fig. 2.—Pallial eye and aesthetes of Acanthopleura spiniger (Moseley). |
Each of the eight valves of the shell is made up of two distinct calcareous layers: (a) an outer or upper called the tegmentum, which is visible externally; (b) a deeper layer called articulamentum which is porcellaneous, quite compact, and entirely covered by the tegmentum. In the lower forms the two layers are coextensive and have smooth edges, but in the higher forms the articulamentum projects laterally beyond and beneath the tegmentum into the substance of the mantle. These projections are termed insertion plates; they are usually slit or notched to form teeth, the edges of which may be smooth and sharp, or may be crenulated. The anterior margin of each valve except the first is provided with two projections called sutural laminae which underlie the posterior margin of the preceding valve.
| From Lankester, Treatise on Zoology. |
| Fig. 3.—Ventral aspect of three species of Polyplacophora showing position of gills. |