Elementary Text-Book of Zoology, Vol. 2: Special Part: Mollusca to Man; With 215 Woodcuts (Classic Reprint) - Livres de poche

Excerpt from Elementary Text-Book of Zoology, Vol. 2: Special Part: Mollusca to Man; With 215 Woodcuts

Bilaterally symmetrical unsegmented animals, without a locomotory skeleton; with a ventral foot and usually a calcareous univalve or bivalve shell; with brain (supraœsophageal ganglia), circumœsophageal ring, and subœsophageal group of ganglia.

Since Cuvier several different groups of animals, which were placed amongst the worms by Linnaeus, have been included in the Mollusca. Of late years, however, the anatomy and development of these forms have been more closely examined, and it seems fairly certain that some of them are allied to the Worms. In any case, the group Mollusca must be looked upon as of more limited extent than has for some time been the case. The bivalved Brachiopoda, which in structure and development stand in closer relationship to the Bryozoa, may be removed from the Mollusca and united with the latter under the head Molluscoidea. The Tunicata also must be constituted an independent group between the Mollusca and the Vertebrata.

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