58 



HYDROZOA 



Sab-Class II. Hydromedusae. 



Order 1. GTMNOBLASTEA-ANTHOMEDUS*. 



( Tuliularia (flg. 35). 

 Fam. 1. Tubularidas .................. J. Hybocodon. 



( Corymorpha (flg. 34). 



- 2-Pennarid* ..................... { Sta. 



( BongainviUia (figs. 36, 37). 

 3. Eudendridai .................. \ Pevigonium. 



(. Lizzia (flg. 44). 



t Cladonema. 



4. Cladonemida: 



Clavatella. 



6. Dicorj'nidse .................. Dicoryne (flg. 40). 



1 Sarsiadse (flg. 45). 



7. Corynidse .................... \ Coryne. 



( Syncoryne (flgs. 41, 46). 



3-Hydractin.d* ............... { > * 39 >' 



. 10.Hy.Wd. ........................ 



Order 2. CALYPTOBLASTKA-LEPTOMEDUS.*. 

 Fam. !. Plumularida, .................. 



/ Eucopidse. 



3. 



4. Thaumantiad*.. 



( Obelia. 

 /Thaumantias. 



] iwSrtmi. 



(Tima. 

 f jEquorea. 

 5. .ajquoridaB ..................... < Zygodaetyla. 



( Rhegmatodes. 

 Order 3. TRACHOMEDUS.S (Haeckel). 



Fam. 1. Petasidae ..................... Petasus. 



2. Trachynemidffi ............... Rhopalonema. 



3. Aglauridae ..................... Aglaura. 



4.Geryonid ..................... { Car'marina (flgs. 48, 49). 



Order 4. NARCOMEDUS^: (FTaeckcl). 



Fam. 1. Cunanthtdae .................. Cunina {figs. 50, 51). 



,, 2. Peganthidse .................. Polyxenia. 



Order 5. HTDROCORALLIS-.S (Moseley). 



Fam. 1. Milleporidce ................... Mlllepora (figs. 52, 53). 



f Sporadopora, 

 2. Stylasteridse .................. ( Distichopora. 



( Astylus (flg. 54). 

 Order 6. S;PHONOPHORA. 



Sub-Order 1. Physophoridse. 



Fara. 1. Athorybiada; ................. Athorybia, 



2. Physoplioridse ............... Physophora (fig. 57, C). 



(ForskalHa. 

 Halistemma. 

 Agalma (flg, 57, E). 

 4. Apolemiadffi .................. Apolemia. 



5. Rhizophysidse .............. Rhizophysa. 



Sub-Order 2. Physalidse. 



Fam 1. Physalidte ..................... Physalia. 



Sub-Order 3. Calycophoridae. 



Fam. 1. Hippopodiidx ................ Gleba. 



( Praya. 

 2.Diphyidas ..................... 1 Diphyes (flg. 67, A). 



(Abyla. 

 3. Monophyidse .................. Sphseronectes. 



Sub-Order 4. Discoideaa. 



ram.l.Velemd 



The Hydrozoa present a greater simplicity of ultimate 

 structure than do any animal organisms possessed of as 

 great a complexity of external form. As in all Metazoa or 

 Enterozoa, the life cycle of a hydrozoon starts with an egg 

 which is at first a single cell or unit of protoplasm, but 

 proceeds after fertilization to multiply by transverse fission 

 in such a way that the resulting cells or units are arranged 

 in two layers, each one cell deep, disposed around a central 

 cavity the enteron or archenteron. The sac thus formed 

 is known as a diblastula (figs. 1, 2, and 25). By the forma- 

 tion 1 of a mouth to the sac, the enteron acquires the functions 

 of a digestive retort in which food matters taken in at 

 the mouth are brought into a chemical condition suitable 

 for the nutrition of the surrounding cells. The two layers 

 of cells (of which the outer only acquires additional layers 2 



1 In HydromeduscE the inner layer of cells forms by delamination, 

 in Scyphomedusce by invagination. In the latter case the sac closes 

 up, and the mouth is formed by a new opening. 



2 It is probable that the numerous rows of cells described in the 

 endoderm of Tvindaria and Corymorpha by Allman, in his great mono- 

 graph of the Tubularian Hydroids, are due to a plication of the 



by the division of the primary cells, and that by no 

 means in all cases) received from Allman (Phil. Trans., 

 1855) the names respectively of the & 



ectoderm and the endoderm, having 

 previously been shown by Huxley 

 (1849) to be the fundamental mem- 

 branous constituents of which the 

 most varied parts of the more com- 

 plex Hydrozoa, such as tentacles, 

 swimming bells, and air-bladders 

 are built up in the adult condition. 

 Huxley also pointed out the iden- 

 tity of these membranes with the 

 two primary layers of the vertebrate 

 embryo. The endoderm and the 

 ectoderm, which present themselves, 

 as is now known, in the diblastula (or 

 gastrula) phase of all Enterozoa, re- 

 main in Hydrozoa (and also in the allied 

 groups of Caelentera) as permanently distinguishable ele- 

 ments of structure. This important disposition is associ- 

 ated with and dependent on the simple character which the 

 archenteron or primitive digestive space retains. Into what- 

 ever lobes or processes the sac-like body may be, so to 



FIG. 1. Diagram of a Di- 

 blastula. a, orifice of in- 

 vagination (blastopore) ; 

 b, archenteric cavity ; c t 

 endoderm ; d, ectoderm. 

 (From Gegenbaur's Ele- 

 ments of Comparative 

 Anatomy.) 



FIG. 2. Formation of the Diblasrnla of Eucope (one of the Calyptoblastic Hydro- 

 medusa) by delamination. (From Balfour, after Kowalewsky.) A, B, C, three 

 successive stages, ep, ectoderm; hy, endoderm; a/, enteric cavity. 



speak, moulded, whether tentacles 3 or broader expansions, 

 into these the cavity of the archenteron is extended in the 

 first instance ; and where the actual cavity is obliterated 

 the endodermic cell-layer remains to represent it (Gefass- 

 platte or endoderm-lamella, see figs. 7 and 16). 



Conversely, whatever canals or spaces are discovered in 

 the substance of a hydrozoon (excepting only the cavity of 

 ectodermal otocysts) are simple and direct continuations 

 of the one original enteric cavity of the diblastula, and all 

 such spaces are permanently in free communication with 

 one another. 4 



The whole of the Hydrozoa seem to present a lower grade 

 of structure than the Actinozoa, in so far as the latter, 

 whilst retaining permanently free communication between 

 ! all parts of the archenteric space, yet exhibit a differentia- 

 tion of this space into an axial and a periaxial portion a 

 digestive tube and a body cavity. The differentiation has 

 only to proceed a step further, namely, to the closure or 

 shutting off of the axial from the periaxial portion of 

 the archenteric space, and we obtain the condition which 

 characterizes the adult forms of the Caelomata, or animals 



original endodermal cell-layer. The two kinds of cells in two layers 

 figured by the same authority in the endoderm of Gemmellaria imjilexa, 

 pi. vii. fig. 5, cannot, however, be thus explained. 



3 Some solid tentacles, with a single axial row of endodermal cells, 

 form an exception to this statement. 



4 The observations of Eilhard Schulze cited in the article COXENTERA 

 do not form any real exception to this statement. 



