HYDROZOA 



73 



The genital glands have the form of wide outgrowths or 

 lamelliforrn enlargements in the course of the radial canals 

 (figs. 48, 49). No hydriforin phase is kuown in any 

 member of this group, and one at least (Geryonia) has been 

 observed to develop from the egg directly into the medusa- 

 form. 



Order 4. Narcomedusve. These have the same characters 

 as the Trachomedwce, excepting that the genital glands are 

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

 growths thereof (figs. 50 and 51). Further, 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 iu this group, 

 whilst ^Egina and jEyinopsis have been shown to develop 

 directly from the egg to the medusa-form. 



FIG. 50. Cunina rhododactyla, one of the Narcomtdtate. c, circular canal; h, 

 ' otoporpae " (ear-rivets) or centripetal process of the marginal cartilaginous 

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

 ing canal (pouch of stomach) ; /f, tentacles ; (IP, 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 therefore), are characteristic of many 2?artoniedus<e and Trachomeduste. 

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

 with the solid marginal ring. 



The two orders Trachomedtisce and Narcomedusce are established 

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

 Carus as Haplomoiyha, and by Allmau as Monopsea. These latter 

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

 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 of the 

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

 as Trachylince with Anthomcdusce and Leptoineduscc, which are 



Fio. 51. Diagram of a vertical section through a young Cunina rhododactyla, 

 passing on the right side thruugh a radiating pouch, b, tentaculocyst ; r, 

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

 springing from a tentaculocyst (otoporpa) ; t. stomach; /, jelly of the disc; 

 r, radiating canal or pouch ; tt. tentacle (solid, cartilaginous) ; lir, tentacle 

 root ; p, velum. (From Gegenbaur.) 



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

 genitalia exists between Anthomedusas and Nareoinedusoe on the 

 one hand and LeptonuduscE and Trachomedusee, 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 Trachonudusce, gave rise by buds 

 from its enteric walls to young Cuninoe (Narcomedusoe, tig. 50), 

 but this has been explained by the observations of Franz Sehulze 

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

 of ciliated Planulos entering the mouth and enteric chamber of the 

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

 supposed internal buds of Cunina observed by Gegenbaur, Fritz 

 Miiller, and iletschnikow. 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 Cunina. The anatomy of these forms 



is fully given in Haeckel's memoirs in the Jcnaische Zeitachrifl, vols. 

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

 Elmer's Jtledusen, 1878. 



Order 5. Hydrocorallinve. These are llydromedwae 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- 

 tacular region free from such 

 hardening. The inedusiform 

 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 

 (Stylasteridtf). No such 



p.ivitif> Jiavp Kopn rlpfpprjvl 

 in Others (Mllleparidce), which 



may, therefore, give rise to 

 complete medusiform persons. 

 In 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 

 definiteness, or in distinct rows (fig. 55). The position m 

 these two kinds of hydriform persons is marked by definite 

 groups of pits (cyclosystems) in the dried calcareous skeleton 

 of the colonies, which simulate the calycles of the stony 

 coral.-; (Anthozoa). 



'G- 52. Portion of the calcareous 

 corallum of Millepora nodota, show- 

 ing the cyclual arrangement of the 



< F n 



FIG. 53. Enlarged view of the surface of a living Millepora. showing five 

 dactylozooids surrounding 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 Stylasteridas previously regarded as Anthozoa to the 

 category of calcigenons hydroids, and founded the order of 

 Hydrocorallinae. The Stylasteridas 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 v 

 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 N orwegian coast. The woodcuts illus- 

 trating the structure of this group are borrowed from Mr Moseley 's 

 Notes of a Naturalist on the " Challenger." 



The nearest allies of the Hydrocorallince are such polymorphic 

 Gymnoblastea as Hydraclinia (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 ord^er. 



