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Show LEC'"fURE II. ON ~ri-IE CLASSIFICArriON OF ANIMALS. 'l'HE HYDROZOA, ACTINOZOA, POLYZOA, BRACHIOPODA, ASUIDIOIDA, LAMELLIBHANCHIA 'l'A, BRANCHIOGAS'l'EROPODA, PULMOGASTJijROPODA, PTEROPODA, AND CEPHALOPODA. IN giving an account of the lowest forms of animal life in tho preceding Lecture, I have substituted for a definition of each class, a description of the structure of some particular member of that class, or of the organic features which are most obviously characteristic of the class; because, in hardly any of those groups has the structure of many, and widely different, members been thoroughly and exhaustively worked out. I entertain little doubt, however, that the main features of the description of Spongilla might substantially be taken as a definition of the Spongida, and those of the description of ParamCEcium, as a definition of the Infusoria. On the other hand, we possess no such ·complete knowledge of the vital cycle of any Gregarina or Rhizopod; and neither description nor definition of the corresponding classes, of a thoroughly satisfactory kind, is attainable. No such difficulties beset us in studying the next class, the HYDHOZOA (comprising the Hydroid polypes and the Medusm), which may be defined with as much precision as any group in the Animal Kingdom. All the Hydrozoa exhibit a definite histological structure, their tissues primarily presenting that kind of organization which has been called cellular. Again, the body always exhibits a separation into at least two distinct layers of tissue- 'l'IIE HYDROZOA. 21 an outer and an inner-which have been t rmed~ respectively, ectoderm and endoderm. Tho ondodenn is that layer which lines tho inner cavities of the bodv, from tho mouth inwards· the ectodenn is that which forms its" external covering. ' Those two layers arc shown in tho ac ompanying diagram- Fig. 5. Fig. 5.-Diagrams illustrative of the mutual relations of the llydrozoa.-1. Ilydt·a. 2. Scrtularian. 3. Diphyes. 4. Physophorid. 5. Lucernm·ia. a. Ectocyst. b. Endocyst. c. Their enclosed cavity. P. Tentacles. N. Nata.torial organ. T. Crenosaxc. B. Bract. C. Cell. S. Polypite or digestive cavity. G. Reproductive organ. A. Air vesicle. F. Float. I., Il., III., IV. represent the successive stages of development of a Merlusiform zooid or reproductive organ. 1natic sections of the leading forms of Hydrozoa, tho ectoderm being represented by the thin line with the adjacent clear . pace, tho endoderm by tho thick dark lin . |
