Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/577

561 HYDROZOA 561 persons may secrete a horny tubular protective case (perisarc), but this does not form cups for the reception of the tentacular crown nor cases enclosing groups of medusi- form buds (gonangia). The fully-developed niedusiform FIQ. 37. Portion of colony of nourjainvillea ( Enrlendrium)fruticosa (AntJiomedusce- calyptoblastea) more magnified. (From Lubbock, after Allman.) persons never possess otocysts nor tentaculocysts, but always ocelli at the base of the tentacles. The latter are usually four or six, corresponding to the same number of simple radial enteric canals, but may be more numerous or reduced to one or to two ; rarely they are branched (Cladonema), F.G. 40. The sexual glands are placed in the wall of the mauubrium, ither equally distributed all round it or in four separate perradial groups, which are often divided into eight ad- radial groups by the perradial longitudinal muscles. This is a very well defined group, since tlie Gymnottastea of Allman, based on the characters of the hydriform persons also known as 2 ubularice and Oymnotoka correspond exactly with the Anthomedusa of Haeckel s new system. Hydra is included here, though placed in a separate order by Allman. Some of the leading ionns of hydriform and niedusiform persons are given in the cuts (figs. 34 to 42). The greatest range in the amount of degenera tion of the medusiform persons is seen even in genera of the same family e.g., Turris and Clava the former producing free medusae, the latter sessile sporosacs. The Oceanidce of Gegenbaur (excluding the Williadcc, which Haeckel assigns to the next group) correspond on the whole to the medusa-forms of this order. Fig. 41. Fig. 42. Fip. 41. Hydriform person of Syncoryne, with medusiform persons budding from it. and shoun in various stages of development, a, b, c, d, e. (From Gegenbaur after Desor.) FIG. 42. Hydra viridis. ov, ovary; te, testis. Order 2. CalyptoUastea-Leptomed iisce. These are Ilydro- medusce of which the hydriform phase is known in a large number of cases, whilst of others only the medusa- forms are known ; none are known to develop directly from the egg to the medusa-form. As in the preceding group, the medusiform persons may reach full development or Fig. 43. Fig. 44. FIG. 43. Diagram of a colony of Campamilaria, showing four forms of per sons. A, portion of a fixed colony ; a, hydriform person ; b, bud-bearing hydriform person (blastostyle) ; B, free -swimming colony, being a sexless medusiform person (blastocheme of Allman), with modified mcdusiform persons budding from its radiating canals, as sporosacs. (After Allman.) FIG. 44. Medusiform person (Lizzia), one of the Ant/iomedttste, detached from a hydroid colony of the family Endendridce. Ocelli are seen at the base of the tentacles, and two medusiform buds on the sides of the manubrium. (After Allman.) xhibit themselves as degenerate sexual sacs on the hydri form colonies. The ectoderm of the hydra-forms always secretes a perisarc which forms a cup-like protection (hydro- theca) to the tentacle-crown, and which also encloses the ?roup of medusa-buds in peculiar horny cases (gonangia). The fully-developed medusiform persons (fig. 47) either