Elements of zoology - 1873 Author:Andrew Wilson Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: CHAPTER IV. CCELESTERATA. General Characters—Classification—Characters of the Hydrozoa —Orders of Hydrozoa. The Codenterata present a marked advance on ... more »Pro- tozoic structure, and the consideration of this sub- kingdom leads us to deal with increased specialisation both of organs and tissues. The Ccelenterate animals represent the Radiate division of the older naturalists ; but from the limits of the newer division, many forms, classified under the Radiate type, have 'been removed and placed in another sub-kingdom of recent construction—the Echinozoa. The term Coelenterata is derived from the consideration of a special feature in the morphology of the group, the signification of the name implying that, as in the higher division of the order, the digestive sac and general cavity of the body are in free communication with each other. A definite " somatic," or body-cavity always exists, the symmetry being typically of the radial kind, united with bilateralism. The body-tissues of the Cceleuterata consist fundamentally of two layers or membranes, the primitive simplicity of which may,however, become more highly specialised, muscular fibres being differentiated in the higher members of the group. To the outer or more external of these membranes, the term " ectoderm " is applied, the inner layer being named the "endoderm" (Fig. 10, 1, c d). Fig. 10. Morphology or Ccslentrrata. Vertical section of Hydrozoon. a, mouth ; 6, tentacles ; c, endoderm ; d, ectoderm. 2. Vertical section of Actinozoon (the letters refer to the corresponding parts in each figure); -s, stomach-sac. 3. Transverse section of Hydrozoon. 4. Transverse section of Actinozoon ; m, mesentery. The vibratile filaments, known as " cilia," are found on both ectoderm and endoderm—the former being most gener...« less