HYDROZOA 



59 



with blood-lymph, space distinct from digestive canal.^ 

 With the attainment of the coelomate condition, the two 

 fundamental cell-layers, ectoderm and endoderm, which still 

 appear in the embryo, become so far interwoven, and their 

 products so.highly differentiated, that it is no longer possible 

 to recognize them as anatomical structures in the adult. 



The only deep-seated distinction between Hydrozoa and 

 Anthozoa (the Actinozoa being thus teriiied when the 

 CtenopJuyra are detached from them) appears to be the 

 particular differentiation of the archenteric space in Anthozoa 

 which has just been noted. It is no longer possible to 

 separate the two groups from one another as Exoarii and 

 Endoarii, as was proposed by Eapp {Ueber die Polypen im 

 Allgemeinen und die Actinien insbesondere, Weimar, 1&29) 

 — the first term indicating the Hydrozoa as possessed of 

 external generative organs, whilst by the latter term the 

 Anthozoa are pointed to as having internal generative 

 organs.^ This distinction breaks down completely in the 

 case of Lucernaria, and even in that of the so-called phanero- 

 carpous and some other medusae which discharge their 

 geni tal products by the mouth, and quite rarely by rupture of 

 the outer body-wall. The tendency to form calcareous 

 deposits in the deep layers of the ectoderm, or mesoderm, 

 as it has been termed, exhibited almost universally by the 

 Anthozoa (whence the name Coralligena applied to them), 

 is distinctive of them, though it has been shown first by 

 Louis Agassiz, and more fully and recently by Moseley, to 

 be paralleled among Hydrozoa, by the external calcareous 

 deposits of the abundant and widely distributed Millepores 

 and Stylasterids. A minute distinction between Hydrozoa 

 and Anthozoa, which does not, however, hold good uni- 

 versally, is found in the form of the barbed threads ejected 

 by the nematocysts, Instead of the complicated forms 

 present in the latter group, the Hydrozoa are usually pro- 

 vided with either an unbarbed thread or one in which the 

 barbs are confined to three at the base and a few minute 

 barblets (fig. 5). 



Fundamental Forms of the Hydrozoa. — -The diblastula 

 derived from the egg of a hydrozoon, when provided with 

 a mouth, may be spoken of (as are the equivalent forms 

 in other animals groups) as a person. Either this person 

 elong&ites and develops tentacles in a circlet around or near 

 the mouth, and usually becomes fixed by the aboral pole of 

 the sac-like body, or the sac gradually assumes the form 

 of a clapper-bell or of an umbrella with greatly thickened 

 handle, the mouth being placed at the free end of the handle 

 or of the clapper, and the animal freely swimming by the 

 contractions and expansions of the dome of the bell (disc 

 of the umbrella). The two forms of persons are known, 

 — the former as the "hydriform" (2, 3 in fig. 16), the 

 latter as the "medusiform" (4, 5, 6 in fig. 16). 



The HYJDEiFORM PERSONS Usually occur as fixed branching 

 colonies or trees (figs. 36 and 37) produced by lateral budding 

 from an original hydra-form developed from a diblastula. 



The hydriform person in its most fully developed state 

 is seen in the colonies of Tuhularia. In such a colony a 

 number of hydriform persons are united like the flowers of 

 a plant on its branches (whence Allman's terms hydranth, 

 hydrophyton). Each hydriform person (fig. 35) has an 

 elongated body with oral and aboral pole. The mouth is 

 placed centrally at the oral pole, which is somewhat enlarged 

 and conical. At the apex of the cone, immediately around 

 the mouth, is a circlet of small tentacles ; at the base of 

 the cone is a second circlet of larger tentacles ; the surface 

 of the oral cone is termed the hypostome. In other genera 



1 The Enterozoa or Metazoa admit of division into two grades — (1) 

 tlie Ccdeniera, including sponges, polyps, jelly-fish, and corals, and 

 (2) the Ccelomata, including all remaining forms. 



' See, however, note to tlie paragraph headed Definition of the 

 Uydrozoa, p. 655. 



{e.g., Hydra, fig. 42) the smaller circle of tentacles is 

 wanting; in others, again, the tentacles are irregularly 

 placed and not concentrated into one circlet (fig. 38). 

 We regard the former as the typical condition. In the 

 hydriform persons of the Scyphomedusos (figs. 26 and 27) 

 the vertical axis is much shortened, the hypostome is flat, 

 and the whole body cup-like or hemispherical. 



The tentacles of the hydriform person are sometimes 

 hollow {Hydra, Garveia nutans, Hydrocorallince), being 

 mere prolongations of the sac-like body ; but usually, 

 though the endodermal cell-layer is continued into them, 

 they are solid (2 in fig. 16). Very generally the tentacles 

 of the hydra-form are indefinite in number, but in those 

 belonging to the group of Scyphomedusce a primary series 

 indicating four radii (perradial) can be distinguished, to 

 which are added four intermediate to these, marking four 

 secondary radii (interradial), whilst eight more placed 

 between the eight of the perradial and interradial series 

 are known as adradjal tentacles. The surface of the hydra- 

 form may be entirely naked, or encased in a horny tube 

 (perisarc) formed by the ectoderm : this may be confined 

 to the aboral portion of the hydranth and to the common 

 stem which unites the persons of a colony, or it may rise 

 up and form a cup (or hydrotheca) around the oral region 

 of the hydranth (figs. 32 and 33), 



The bodies of all hydriform persons, as well as the ten- 

 tacles, are excessively contractile, and when hydrothecae are 

 present can be withdrawn into them. 



The ectoderm or outer cell-layer furnishes the protective 

 and contractile tissues of the hydra-form. Very usually 

 it is not more than one or 

 two cells deep, and is sepa- 

 rated from the endoderm by 

 a structureless lamella of 

 firm consistence. InHydra 



large cells of the ectoderm F„5.3._Epiaermo-muscularcell8ofJs,dra. 

 (neuro-muscular cells of m, muscular -flbie processes. (After 

 Kleinenberg) bound the Kleineuberg, from Gegenbam-,) 



external surface (fig. 3) and give off horizontal muscular 

 processes which lie side by side on the structureless lamella- 

 forming thus a deep muscular coat, the fibrous elements of 



Fio 4.— Portion of the body-wall of Hydra, showing ectoderm cells above, 

 separated by "structureless lamella" from three flagellate endoderm cells 

 below The latter are vacuolated, and contain each a nucleus and several dark 

 granules. In the middle ectoderm cell are seen a nucleus and three nemato- 

 cysts, with trigger hairs projecting beyond the cuticle. A large nematocyst, 

 with everted thread, is seen in the right-hand ectodennal ceU. (After F. E. 

 Schulze.) 



which are not independent cells. In larger species some of 

 the fibres may become separated from the tegumentary or 

 superficial cells, and acquire the character of independent 

 nucleated corpuscles {Hydractinia, Van Beneden). No 

 nervous elements nor sense-organs occur in any hydra-form 

 (except perhaps the Jyucernarice), In Antennularia some 

 ectoderm cells are amcebiform, and project processes which 

 change shape (nematophors). Tactile hairs (palpocils). 



