86 



CCELENTERATA 



HALISARCA Dujardin. Body small, soft and irregular, with large 

 oscula somewhat elevated: several species. 



H. dujardini Johnston. Small, pale-yellow, gelatinous sponges grow- 

 ing on red algae: in 5 fathoms, off Rhode Island. 



SUBPHYLUM 2. CNIDARIA.* 



Jellyfish, hydroids, corals, etc. Aquatic animals, either sessile or 

 free-swimming, in which the body possesses a single internal cavity, the 

 gastrovascular space (Fig. 131). This has usually a single opening to 

 the outside, which is called the mouth, and is the common digestive and 

 circulatory cavity; in the simplest cases it is cylindrical in shape, but in 

 the higher and larger forms is much branched, 

 forming a system of canals. 



The cnidarians are predacious animals and 

 usually possess long vibratile tentacles by means 

 of which they take their prey. These tentacles, 

 as well as other parts of the body, are provided 

 with numerous characteristic organs of peculiar 

 structure called the nettle organs or nematocysts 

 which render them effective instruments in the 

 performance of this important function. A nettle 

 organ (Fig. 145, 3) consists of a spiral, thread-like 

 tube with several barbs at the base which lies 

 coiled within a cavity in a specialized cell called 



a cnidoblast (1). The cavity is filled with a poisonous fluid; its walls form 

 an ovoid sac, the outer end of which is continuous with the thread-like tube. 

 A minute spine, the cnidocil (2), projects from the free surface of the 

 cnidoblast into the water and when the surface of the ectoderm is irri- 

 tated, either by actual contact or in other ways, the tube is shot out with 



* See "Contributions to the Natural History of the U. S.," Vol. 3 and 4, by L. 

 Agassiz, 1862. "Invertebrate Animals of Vineyard Sound," by A. E. Verrill, Rep. U. S. 

 Fish. Com., 1871. "Les Coelenteres," by Delage et He'rouard, Traite" de Zoologie 

 concrete, Vol. 2, 1901. "Hydroids of the Woods Hole Region," by C. C. Nutting, 

 Bull. U. S. Fish. Com., Vol. 19, 1899. "Synopsis of North American Invertebrates, 

 The Hydromedusae," by C. W. Hargitt, Part I, II, III, IV, Am. Nat., Vol. 35, pp. 

 301, 379 and 575, 1901, and Vol. 37, p. 331, 1903. "The Hydroids of the Pacific Coast of 

 North America," by H. B. Torrey, Univ. of Cal. Pub., Vol. 1, p. 1, 1902. "The Medusae 

 of the Woods Hole Region," by C. W. Hargitt, Bull. Bur. Fish., Vol. 24, p. 21, 1904. 

 "Notes on Crelenterates of Woods Hole," by C. W. Hargitt, Biol. Bull., Vol. 14, p. 95. 

 "A Synopsis of the Fixed Hydroids of New England," by J. S. Kingsley, Tufts College 

 Studies, Vol. 3, p. 13, 1910. "Medusae of the World," by A. G. Mayer, 1910. "The 

 Hydroids of the West Coast of North America," by C. M. Fraser, Bull, from the Lab. 

 of Nat. Hist, of Univ. of Iowa, Vol. 6, 1911. "Some Hydroids of Beaufort, North 

 Carolina," by C. M. Fraser, Bull. Bur. Fish., Vol. 30, p. 337, 1912. "A Biological 

 Survey of the Waters of Woods Hole and Vicinity," by F. B. Sumner and others, Bull. 

 Bur. Fish., Vol. 31, 1913. 



'-'I 



Fig. 145 Diagram of 

 a nettle organ in the ec- 

 toderm of a cnidarian 

 (Lendenfeld). 1, cnido- 

 blast; 2, cnidocil; 3, 

 nematocyst ; 4, nerve 

 fibre; 5, tactile cell. 



