HYDROZOA 



73 



The genital glands have the form of wide outgrowths or 

 lamelliform enlargements in the course of the radial canals 

 (figs. 48, 49). No hydriform phase is known in any 

 member of this group, and one at leasb {Geryonia) has been 

 observed to develop from the egg directly into the medusa- 

 form. 



Order 4. Nareomedusoe. — These have the same characters 

 as the Trachomedusce, excepting that the genital glands are 

 in the wall of the manubrium or in pocket-like radial out- 

 growths thereof (figs. 50 and 51). Farther, the marginal 

 tentacles of the disc possess peculiar " roots," which can be 

 traced upwards into the gelatinous substance of the body. 

 No hydriform phase has been observed in this group, 

 whilst JEgina and ^ginopds have been shown to develop 

 directly from the egg to the medusa-form. 



Fig. 50. — Ounina rhododactyla, one of the Narcomedusce. c, circular canal ; h, 

 " otoporpsB " (ear-rivets) or centripetal process of the marginal carUlaf;inous 

 ring connected with tentaculocyst; £, stomach; /, jelly of the disc; r, radiat- 

 ing canal (pouch of stomach); ^£, tentacles ; £w, tentacle root. (After HaeckeL) 

 The lappets of the margin of the disc, separated by deep notches, above 

 which (nearer the aboral pole) the tentacles project from the disc (not mar- 

 ginal tlierefore), are characteristic of many Narcomeduscs and Trachmneduste. 

 Cartilaginous strands (the mantle rivets or peronise) connect the tentacle root 

 with the solid marginal ling. 



The two orders Trachomed-asce and NarcoTnedusm are established 

 by Haeokel in his new " system " for the peculiar forms classed by 

 Carus as Baplomm-pha, and by Allman as Monopsea. These latter 

 names have reference to the fact that no hydriform phase is known 

 to occur in the life-history of these organisms, a fact which is not 

 peculiar to them, and, if it should prove to be not universal amongst 

 them, would by no means invalidate their claim to a distinct posi- 

 tion on the grounds afforded by the characters above given. They 

 are remarkable for a certain hardness and stiffness of the gelatinous 

 substance of the disc, or at any rate of the cellular axis o£ the 

 tentacles, on accout of which the orders are contrasted by Haeckel 

 as Trachylince with Anthomedusce and Leptomedusce, which are 



Fig. 91.— Diagram of a vertical section through a young Ounina rlwdodcuityla, 

 passing on the right side through a radiating pouch. 6, tenlaculocyst; c, 

 circular canal; gr, ovary; h, marginal cartilage and connecting process 

 springing from a tentaculocyst (otoporpa) ; h, stomach; I, jelly of the disc; 

 r, radiating canal or pouch; tt, teutacl^ (solid, cartilaginous); tw, tentacle 

 root; V, velum. (From Gegenbaui'.) 



termed LeptoUnce ; a curious parallelism as to the position of the 

 genitalia exists between Anthomedusoe and Nareomedusoe on the 

 one hand and Zeptoriiedusce and Trachomedusm, on the other. 

 The orders present a very high degree of development, both in 

 coarser and histological differentiation. At one time it was sup- 

 posed, in accordance with Haeckel's observations, that Geryonia 

 (Carmarina, fig. 48), one of the Trachomedusoe, gave rise by buds 

 from its enteric walls to young Cunince (Ndrcomedtisce, Hg. 50), 

 but this has been explained by the observations of Franz Schulze 

 and of Uljanin as due to parasitism, young Cunince in the condition 

 of ciliated Planulce entering the inouth and enteric chamber of the 

 Carmarina. The same explanation probably applies (Claus) to the 

 supposed internal buds of Gwnina observed by Gegeubaur, Fritz 

 Miiller, and Metschnikow. The process is sufficiently remarkable 

 according to the last observer, for the first generation of buds pro- 

 duce a second generation by external gemmation, before attaining 

 the characters of the parent Cunvna. The anatomy of these forms 



is fully given in Haeckel's memoirs in the Jenaische Zeitschrifl, vols. 

 i. and ii., 1864-66 ; also further details as to Carmarina are given in 

 Elmer's Medusen, 1878. 



Order 5. Hydrocorallince. — These are Hydromedusce in 

 which the hydriform phase forms large colonies, presenting 

 a copious calcareous deposit 

 in the ectodermal tissue (cce- 

 nosteum of Moseley), leav- 

 ing only the hydranths or ten- 1 

 tacular region free from such 

 hardening. The medusiform 

 persons are, at present, only 

 known in the degenerate 

 form of sporosacs, which 

 occupy cavities (ampullae 

 of Moseley) in the har- 

 dened base of the colony 

 {Stylasteridw). No such 

 cavities have been detected ^"'- ^?— ^°'''jj" °' "^^ calcareous 



1 mr'tt 't \ 1 • 1 corallum of jm^/epora notfosa, show- 



in others (MlUepondm), which Ing the cyclical arrangement of the 



may, therefore, give rise to rySUhTT^v^e'tL;Ea7:;z°e: 

 complete medusiform persons, (f™™ Moseley.) 

 lu all a marked polymorphism has been observed (fig. 53), 

 consisting in the differentiation of longer tentacle-like 

 persons (dactylozooids) and shorter mouth-bearing persons 

 (gastrozooids). The persons of both kinds are either 

 scattered irregularly or the dactylozooids are arranged 

 around the gastrozooids in cyclosystems of greater or less 

 deiiniteness, or in distinct rows (fig. 55). The position oi 

 these two kinds of hydriform persona is marked by definite 

 groups of pits (cyclosystems) in the dried calcareous skeleton 

 of the colonies, which simulate the calycles of the stony 

 corals (Anthozoa). 



Fig. S3. — Enlarged view of the surface of a living MiU^ora, showing five 

 dactylozooids suiTounding a central gastrozooid. (From Moseley.) 



Louis Agassiz was the first to recognize the true nature of the 

 Milleporidce, and his imperfect observations have been fully con- 

 firmed and greatly extended by Mr Moseley {Phil. Trans. , 1878) who 

 added the Stylasteridce previously regarded as Anthozoa to the 

 category of calcigenous hydroids, and founded the order of 

 Hydrocorallince. The Stylasteridce differ from the Milleporidce in 

 possessing a calcified axial style at the base of the dilated portion of 

 each gastrozooid, and further in the ascertained development of 

 sporosacs, and in the greater complication of their cyclosystems. 

 These forms are abundant in tropical seas, and contribute with the 

 Anthozoa and Corallines to the formation of coral reefs. Allopora 

 and Stylaster occur off "the Norwegian coast. The woodcuts illus- 

 trating the structure of this group are boiTowed from Mr Moseley'a 

 Notes of a Naturalist on the " Challenger." 



The nearest allies of the Hydrocorallinoe are such polymorphic 

 Gymndblastea as Hydractinia (fig. 39) ; the definite division of labour 

 and the polymorphism in the former, together with their calci- 

 genous peculiarity, entitle them to rank as a distinct order. 



