HYDROZOA 



69 



of the pile, which has led to diverse accounts of the mode 

 of development of the ephyrre. 



Whilst changes are going on in the configuration of the 

 margin of the disc of an ephyra on its way to the perfect 

 form of the adult Aurelia, the enteric cavity has also under- 

 gone most important changes. Foremost in importance is 

 the development of a single gastral filament on each of the 

 four gastral ridges which necessarily are present in the 

 transverse slice (so to call it) of a scyphistoma, which 

 becomes an ephyra (fig. 26). These rapidly increase in 

 number as the ephyra grows. Further, the enteric cavity 

 at first follows the outline of the ephyra, sending a process 

 into each arm, but then by adhesion of its walls is converted 

 into a four-lobed central chamber, a marginal canal, and an 

 endoderm lamella. A system of canals, the arrangement of 

 which is seen in figs. 29 and 31, subsequently opens out again 

 certain lines and tracts of the conjoined endoderm walls. 



In the adult Aurelia we find the mouth surrounded by 

 four large arm-like perradial processes (figs. 25 and 29) 

 (not tentacles), and leading through a short manubrium 

 into a flattened four-lobed chamber, the lobes being inter- 

 radial, and having on their oral floor numerous gastral 

 filaments (rich in thread cells) (6 in fig. 16). Each pouch 

 or lobe gives off a canal, which runs towards the circular 

 canal at the margin of the disc, but breaks up into three or 

 four secondary canals on its way. Between the pouches 

 come off eight other " radiating " canals (adradial), which 

 do not branch, but go straight to the circular canal. 



The oral lloor of the concavity of each lobe of the enteric 

 cavity is occupied by a horse-shoe-shaped frill (fig. 29, ov), 

 either testis or ovary (the sexes being in separate indi- 

 viduals). The open arms of the horse-shoe are turned 

 towards the centre of the disc, and the folds of the genital 

 frill are so deep as to show themselves on the outer ecto- 

 derrnal wall of the disc. Here, however, there is a very 

 remarkable arrangement, which has rarely, if ever, been 

 correctly described and figured in our common Aurelia. 

 The gelatinous substance of the disc is hollowed out on 

 that part of the oral face corresponding to the position of 

 the genital frills, so as to form four separate extensive pits 

 or chambers. Each of these sub-genital pits has in Aurelia 

 a small round opening on the oral face of the disc (fig. 28, 

 GP), but is otherwise entirely closed, having no com- 

 munication with the genital tissues, from which it is 

 separated by a delicate layer of ectoderm (6 in fig. 16). 

 The pits probably serve to admit water for respiratory pur- 

 poses into close proximity with the genital tissues. 



The whole enteric surface, including canals, is ciliated, 

 whilst the ectoderm is not ciliated, but provided with 

 groups of nematocysts. 



The tentaculocyst in the adult Aurelia is relatively an 

 extremely minute body, completely hidden by the two large 

 marginal lappets (fig. 30, T). Above it (that is, on the 

 aboral surface, as the Aurelia swims) is a deep pit (A), 

 Schafer's fovea nervosa superior, sunk in a sort of bridge 

 which connects the two lappets and overhangs the tenta- 

 culocyst. A similar pit (the fovea inferior) exists on the 

 oral surface. These have been recognized by Clans, Einier, 

 and the Hertwigs as olfactory organs. The tentaculocyst 

 is seen in section in fig. 30 (right-hand figure), which ex- 

 hibits its central cavity continuous with the enteric cavity, 

 its ectodermal pigment spot (eye), and its endodermal mass 

 of concretions (auditory organ). 



The chief muscular mass of Aurelia, except that of the 

 oral arms, is a circular zone on the oral face of the disc. 

 The muscular fibres are not distinct cells, but transversely- 

 striated processes of the epidermic cells (epidermo-muscular 

 cells) (fig. 9). In the " arms " of other medusae, and pre- 

 sumably of Aurelia, the muscular fibre is formed by inde- 

 pendent nucleated cells (fig. 9). 



The nerve-epithelium from the olfactory pits of Aurelia is 

 drawn in lig. 1 4. Starting from this and from the cells of 

 the teutaculocysts are nerve-fibres, which spread themselves 

 on the surface of the circular muscular zone in the neigh- 

 bourhood of the tentaculocysts, and these are connected each 

 and separately with large isolated nerve-ganglion cells (fig. 

 15). The nerve-fibre is continued beyond the cell, and 

 in some instances has been traced into a broadened ex- 

 pansion lying on a muscular fibre (Schiifer). The nerve- 

 ganglion cells lie very superficially immediately below the 

 Hat epithelium of the body surface and between it and its 

 muscular processes. 



The ova and spermatozoa of Aurelia develop in the 

 genital frills from endoderm cells in separate individuals. 

 They pass to the exterior through the mouth. 



Order 3. Conomedusce, Scyphomedusce with only four 

 tentaculocysts, and these perradial. A broad velum (so- 

 called pseudo-velum) of complete circular form is present, 

 differing from that of the HyJromeJnst.i' in the fact that it 

 is penetrated by canals of the enteric system (Charybdcea). 

 The whole umbrella is bell-shaped. The genital organs are 

 four pairs of lamellifonn ridges (fig. 22) which are attached 

 to the four narrow interradial septa that divide the large 

 enteric cavity of the umbrella into four perradial gastro- 

 canal pouches. The lanielliform genital glands hang freely 

 in these pouches. At the edge of the umbrella are four 

 interradial lappet-like prolongations of the gelatinous sub- 

 stance of the disc, which support each a long tentacle (fig. 

 20). The nerve-ring is complete, like that of the Hijdro- 

 meduscv. 



There is now no doubt that Charybdcea, which has been placed in 

 each of the two large divisions of the Hydrozoa, must be classed 

 with the Scyphomedusce. The recent investigations of Glaus 

 (Arbcitcn aim <A m Zool. Jnstititt :.u Il'icn, Bd. i. Hf't. ii., 1878), as 

 well as those of Haeckel and Fritz Miiller, lead to this conclusion. 

 The term Conomcdusic is adopted from Haeckel, who places here, 

 besides Charybdim and Tamoya, other forms, a fuller description of 

 which may be expected in his forthcoming System tier Meduscn. In 

 many respects its quadrangular form, its marginal lappets, its 

 broad enteric pouches in place of fine canals, its vascular velum, and 

 its highly complicated tentaculocysts (fig. 13, tyCltni-yMaa 

 is peculiar. The simplicity of the enteric system and the arrange- 

 ment of the genital glands bring it near to Luccrnaria. The ex- 

 istence of four interradial groups of gastral filaments, and the dis- 

 position of the paired genital glands at the sides of the interradial 

 septa, determine its position to be among the Scyphomedusce. Its 

 development is not known. Figs. 20 to 23 illustrate the structure 

 of ChntrybdaBd, 



Order 4. Peromedusae, Seyphomedusce with four inter- 

 radial tentaculocysts. The enteric system consists of three 

 divisions, an aboral main stomach with four interradial 

 gastral ridges and filament groups; a mid-stomach, which 

 communicates by means of four perradial slits with a very 

 large ring-sinus (occupying two-thirds of the umbrella) ; and 

 thirdly, an oral portion or pharynx, with four wide per- 

 radial pouches. The genital organs are four pairs of 

 sausage-shaped interradial ridges lying on the oral floor of 

 the ring-sinus. 



This is a new group founded by Haeckel, of which we have at 

 present no further details. 



Sub-class II. HYDROMEDUS.E. These are Hydrozoa devoid 

 of gastral filaments ; the sexual persons are always medusi- 

 form, the genital glands are developed sometimes from ecto- 

 dermal cells, sometimes from endoderm, and are always per- 

 radial (in the radii of the first order). The medusiform per- 

 sons always possess a muscular non-vascular velum (hence 

 Craspedota) and a complete nerve-ring (hence Cycloneura of 

 Eimer). The marginal sense-organs are either ocelli or oto- 

 cysts or tentaculocysts. The diblastula, in all cases as yet 

 observed, is formed by delamination (Balfour). The sexual 

 medusiform persons may develop directly from the egg, but 

 more usually the etrg gives rise to a hydriform person the 

 hydroid which difl'ers from a scyphistoma in its elongate 



