MOLLUSCA or Molluscs.
Mollusc is a term derived from the Greek word peactxti;, soft ; and molluscs are animals whose bodies are soft, but are mostly protected by an external shell.
The molluscs are related to the zoophytes by the affinity of their simpler forms, and the higher classes of them to the fishes.
Distribution.—The ocean mollusca of the East Indies have an uninterrupted area from the Red Sea and east coast of Africa to Easter Island in the Pacific, and from Australia to Japan. They have some peculiar shells, but the genera through out are in many instances identical ; even a con siderable number of the same species have been found throughout .the region, and their general character is the same. Mr. Cuming obtained more than 100 species from the east coast of Africa identical with those collected by himself at the Philippines. Of the numerous mollusca of the Red Sea, only a very few are common to the Mediterranean, from which it would seem that these seas have communicated since the first appearance of some existing shells. Meleagriva inargaritifera, or the pearl oyster, Avicula, occurs in the Persian Gulf, on the Madagascar coast, in the Straits of Menaar, Torres Straits, at the Society Islands and Philippines.
Surgeon-General Balfour, when forming the Government Central Museum at 'Madras, and the Mysore Museum at Bangalore, arranged the mollusca and the catalogue of them in accordance with Mr. S. P. Woodward's Manual, a fodrth edition of which was published by Mr. Tato in 1880. The shells in the Madras Museum were collected from every part of the Indian Ocean, the Arabian Archipelago, and Australasian seas. Dr. Train in Journal Indian Archipelago, and Dr. Cuthbert Collingwood in Rambles of a Naturalist, have furnished useful information.
Mr. Benson of 'alto Bengal Civil Service has been a large'eontributor to scientific journals, and be described Dr. Cantor's Chusan shells. Since then the molluscs of the East Indies has been treated in Lieut.-Colonel H. H. Godwin-Austen's Land and Fresh - water Mollusea of India ; Geoffroy Nevill gave a Hand List of the Molluscs in the Calcutta Museum ; Mr. Theobald printed a Catalogue of the Recent Shells in the Bengal Asiatic Society's Museum ; Mr. Theobald and
Mr. Hanley's Conchologia Indica, and Mr. Theobald's Catalogue of the Land and Fresh water Shells of British India, have been the most important works for the Indian area. Lovell Reeve's work on the Molluscs extended to twenty volumes ; and as a recent general work, Mr. S. P. Woodward's Manual of the Molluscs is of great value.
Writing in 1850, Mr. Woodward said that at least 15,000 fossil species of molluscous animals and 12,000 recent were then known. The num bers of living species have since been estimated at 16,792, viz. Cephalopoda, 190 • Gasteropoda, 13,146; Pteropoda, 79; Brachiopoda, 75; • Conclii fera, 3150 • Tunicata, 150. Of 'these 5200 are animal feeders, 3255 vegetable feeders, 3376 in fusorial feeders, and 4960 pulmonifera.
Propagation.—For the continuance of these creatures, the sexes are distinct in the most highly organized (or dicecious) molluscs; they are united in the (moncecious) land pteropods, opistho branchs, and in some of the conchifers. The moncecious land-snails require reciprocal union ; the limneidx unite in succession, forming floating chains. In a few species of gasteropods, a kind of viviparous reproduction happens through the retention of the eggs in the oviduct until the young attain a considerable growth.
The egg clusters of the Teuthidm, calamaries, or squids, have been estimated to contain 40,000 eggs. The Iauthina ocean-snail secretes a raft from its foot, which serves to float its egg-capsules. The spawn of some species consists of large num bers of eggs adhering together in masses, or spread out in the shape of a strap or ribbon, upon which the eggs are arranged in rows. This ribbon is sometimes coiled up like a watch - spring, and attached by one of its edges. The Iauthinm are gregarious in the open sea, and feed on the small blue acalephm (Velella). In rough weather, their floats are broken and detached, and the beach is then strewed with the shells. When handled, a violet fluid exudes from beneath the margin of the mantle. The egg-raft is too large to be with drawn within the shell, and their eggs are attached to its under-surface.