Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/579

563 HYDROZOA 563 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 least (Geryonia) has been observed to develop from the egg directly into the medusa- form. Order 4. Narcomedusac. These have the same characters as the Trachomedusce, excepting that the genital glands are in the wall of the rnanubrium or in pocket-like radial out growths thereof (figs. 50 and 51). Further, the marginal tentacles of the disc possess peculiar &quot; roots, 5 which can be traced upwards into the gelatinous substance of the body. No hydriform phase has been observed in this group, whilst ^Egina and JEginopsis have been shown to develop directly from the egg to the medusa-form. FIG. 50. Cunina rhododactyla, one of the Narcomedusce. c, circular canal; h, &quot; otoporpa! &quot; (ear-rivets) or centripetal process of the marginal cartilaginous ring connected with tentaculocyst; k, stomach; /, jelly of tiie disc; r, radiat ing canal (pouch of stomach); tt, tentacles; tw, 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 Narcomedmce and Trachomcdusx. Cartilaginous strands (the mantle rivets or peronias) connect the tentacle root with the solid marginal ring. The two orders Trachomedusce and Narcomedusac are established by Haeckel in his new &quot; system &quot; for the peculiar forms classed by Carus as Haplomorpha, 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 of the tentacles, on accout of which the orders are contrasted by Haeckel as Trachylincc with Anthomedusae and Leptomedusce, which are FIG. 51. Diagram of a vertical section through a young Cunina rhododactyla, sssing on the right side through a radiating pouch, b, tentaculocyst; c, circular canal; &amp;lt;j, ovary; h, marginal cartilage and connecting process springing from a tentaculocyst (otoporpa) ; k, stomach; I, jelly of the disc; r, radiating canal or pouch; tt, tentacle (solid, cartilaginous); tw. tentacle root; v, velum. (From Gegenbaur.) termed ^Lcplolinw ; a curious parallelism as to the position of the genital la exists between Anthomcdusne and Narcomcdusce on the one hand and Leptomedusce and Trachomedusce, on the other. Hie 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 Trachomedusce, gave rise by buds from its enteric walls to young Cunince (Narcomedusce, fig. 50), but tins 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 mouth and enteric chamber of the armarina. The same explanation probably applies (Claus) to the supposed internal buds of Cunina observed by Gegenbaur, Fritz Muller, and Metschnikow. The process is sufficiently remarkable according to the last observer, for the first generation of buds pro- uuce 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 Jcnaischc Zcitschrift, vols. i. and ii., 1864-66 ; also further details as to Carmarina are given in Limer s Mcdusen, 1878. Order 5. Hydrocorallince. These are Ilydromedusce in which the hydriform phase forms large colonies, presenting a copious calcareous deposit in the ectodermal tissue (ere- nosteum of Moseley), leav- ing only the hydranths or ten- 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 (Stylasteridce). No such cavities have been detected FlG - 52 -~ Portlnn of the calcareous ,-, /Tir-77 -7 1-1 corallum of Millepora nodosa, show- in others (Millepondce), 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 01 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 corals (Anthozoa). ing the cyclical arrangement of the pores occupied by the &quot;persons&quot; or hydranths. Twice the natuial size. (From Moseley.) FIG. 53. Enlarged view of the surface of a living MiUepora, showing five dactylozooids surrounding a central gastrozooid. (From Moseley.) Louis Agassiz was the first to recognize the true nature of the Millcporidce, 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 Millcporidce 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 borrowed from Mr Moseley s Notes of a Naturalist on the &quot; Challenger.&quot; The nearest allies of the Hydrocorallince are such polymorphic Gymnollastea as Hydractinia (fig. 39) ; the definite division of labour and the polymorphism in the former, together with their calci genous peculiarity, entitle them to rauk as a distinct order.