68 



HYDROZOA 



D). Four new tentacles, those of the intermediate or 

 secondary radii, qow appear between the first four, and 

 are termed interradial. At the same time four longi- 

 tudinal ridges grow forward on the wall of the enteric 

 cavity (fig. 26). These interradial ridges have sometimes 



Fie. 28.— Surface view of the BUb-umbrella or oral aspect of Auretia awita, to 

 show the position of the openings of the snb-genitai pits, GF. In the centre 

 is the mouth, with four peiTadlal arms corresponding to its angles (compai-e 

 fig. 26). The four suh-genital pits are seen to be intenadial. x indicates tlie 

 outline of the roof (aboral limit) of a sub-genital pit; y, the outline of its floor 

 or oral limit, in which is the opening (compare 6 of fig. 16). 



been erroneously described as containing each a longitudinal 

 canal connected with a circular canal at the base of the 

 tentacles. They are in reality solid, as is the margin of the 

 hypustome from which the tentacles spring. It is in con- 

 nexion with these four 

 ridges that the gastral 

 filaments will subse- 

 queutly appear, as also 

 the genital organs either 

 along their middle line 

 or adradially to them. 

 The ridges correspond 

 to the mesenteries of 

 the Anthozoa. Eight 

 additional tentacles 

 placed one on each side 

 of the perradial ten- 

 tacles (or of the inter- 

 radial, according as we 

 may choose to regard 

 the matter) next appear, 

 and are distinguished as 

 adradial. All the ten- 

 tacles reaching an equal 

 size, we obtain the ap- 

 pearance seen in fig. 26, 

 when the young scyphi- 

 stoma is looked at from 

 above. Looked at from 

 the side, with its wide 

 hypostome and short 

 vertical axis, the scy- 

 phistoma differs widely from an ordinary hydra-form, and 

 approaches the medusa-form, to which its four longitudinal 

 gastral ridges further assimilate it. The little creature is 

 now about an eighth of an inch in height ; in other genera, 

 but not in Ghrysaora, it may now multiply by the produc- 

 tion of a few buds from its fixed basal disc. After nourish- 

 ing itself for a period, and increasing to four or five times 

 the size just noted, the vertical axis elongates and a series 

 of transverse constrictions appear on the surface, marking 

 off the body of the scyphistoma into a series of discs 

 (figs. 26 and 27), each of which by the development 



Fig. 29. — ^Half of the lower surface of Awrelia 

 auHta. The transparent tissues allow the 

 enteric cavities and canaJs to be seen through 

 them, a, marginal lappets hiding tentaculo- 

 cysts; 6, oral arms; v, axial or gastric portion 

 of the enteric cavity; (?w, radiating and ana- 

 stomosing canals of the enteric system; ov, 

 ovaries. The gastral filaments near to these 

 are not drawn. (From Gegenbaui'.) 



of tentacles and completion of the constriction will become 

 a separate medusa (in its young state called " ephyra "). 

 The tentacles of the Aurelia and the structure of the 

 margin of its hypostome are very different from those of 

 the scyphistoma. They are exhibited in their earliest 

 condition (when the Aurelia-medaaa, is first liberated from 

 its attachment and is an ephyra) in fig. 26, E, F. The 

 margin of the hypostome b drawn out into eight arms 

 (which are not to be confused with tentacles) ; the end of 

 each arm is bifid, carrying a pair of lappets — the marginal 

 lappets which persist in the adult (see figs. 30 and 31). Be- 

 tween the lappets is placed a short and peculiar tentacle, the 

 tentaculocyst or sense-organ. The eight arms of the disc 

 and their tentaculocysts are perradial and interradial. As 

 the organism grows, a set of eight adradial tentacles appear 

 in the notches between the eight arms, but never attain any 

 relatively large size in Aurelia. The asteroid arm-bearing 



&id. 



Fig. 80.— Tentaculocyst and marginal lappets of Aurdia awrita. In the left- 

 hand figure — MLt marginal lappets; T, tentaculocyst; A, superior or aboral 

 olfactory pit ; MT^ marginal tentacles of the disc. The view is from the aboral 

 surface, magnified about 50 diameters. In the right-hand figure — A^ superior 

 or aboral olfactoiy pit; B, inferior or adoral olfactory pit; H^ biidge between 

 the two marginal lappets forming the hood ; T, tentaculocyst ; End, endoderm ; 

 Ent, canal of the enteric system continued into the tentaculocyst; Con, endo- 

 dermal concretion (auditory) ; oc, ectodermal pigment (ocellus). Ihe drawing 

 represents a section, taken in a radial vertical plane so as to pass through the 

 long axis of the tentaculocyst. (After Eimer.) 



character of the margin of the disc is soon obliterated by 

 the relative growth of the intermediate adradial areas, which 

 become quite filled up, so that in the adult the tentaculocyst 

 is carried in a notch instead of on a prominence, and is 

 concealed by the two lappets 

 (figs. 28 and 30). The margin 

 of the disc between adjacent 

 pairs of lappets gives rise to 

 a fold which grows inwards 

 (toward the mouth) during 

 an early stage (fig. 31), and 

 numerous small tentacles (the 

 fringe) appear along the 

 margin of the disc, which 

 soon equal in size the first 

 adradial tentacle. The in- 

 growing fold is the velum or 

 "pseudo-velum," and never 

 increases in size, so that in 

 the adult it is not observ- 

 able. The tentacles also remain very small and fine in 

 Aurelia, forming a continuous fringe along the edge of 

 the disc, interrupted only by the eight notches for the 

 tentaculocysts (fig. 29), 



The sixteen tentacles of the scyphistoma are necessarily 

 attached to the most anterior of the pile of medusae ; they 

 atrophy, but to what extent they may be metamorphosed 

 to form the parts of the ephyra or young medusa has not 

 been determined. The scyphistoma, having given rise to 

 its pile of ephyrae, may (in some genera, Aurelia 'i) 

 redevelop its own kind of tentacles below the constriction 

 marking off the last ephyra. Hence scyphistoma tentacles 

 appear sometimes at the top and sometimes at the bottom 



Fig. 81.— Part of the margin of the disc 

 of a young Aurelia, to show the rudi- 

 mentary velum, Vel, extending from 

 the marginal lappets, ML, on either 

 side; T, the small tentacles fringing 

 the disc. 



