68 



HYDROZOA 



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

 secondary radii, now 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 



TO: 



C.P. 



It is in con- 



FIG. 28. Surface view of the sub-umbrella or oral aspect of Aurelia aurita, to 

 show the position of the openings of the sub-genital pits, OP. In the centre 

 is the mouth, with four perradial arms corresponding to its angles (compare 

 fig. 26). The four sub-genital pits are seen to be interradial. x indicates the 

 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 

 hypostome from which the tentacles spring, 

 nexion with these four 

 ridges that the gastral 

 filaments will subse- 

 quently 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 Chrysaora, 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 Aurelia 

 aurita. The transparent tissues allow the 

 enteric cavities anil canals to be seen through 

 them, a, marginal lappets hiding tentaculo- 

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

 of the enteric cavity; ge, radiating and ana- 

 stomosing canals of the enteric system; on, 

 ovaries. The gastral filaments near to these 

 are not drawn. (From Gegenbaur.) 



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-medusa, is first liberated from 

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

 margin of the hypostome is 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 



FIG. 30. Tentaculocyst and marginal lappets at Aurelia aurita. In the left- 

 hand figure ML, 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 olfactory pit; B, inferior or adoral olfactory pit; //, bridge between 

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

 Ent, canal of the enteric system continued into the tentaculocyst; t'yn, endo- 

 dermal concretion (auditor)'); c, ectodermal pigment (ocellus). The drawing 

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

 long axis of tlie tentaculocyst. (After Elmer.) 



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- 



. , FIG. 31. Part of the margin of the disc 



growing fold IS the velum or ofayoungXiuv/fa, to show the rudi- 

 " naonrln vpliim " anH npvpr mentary velum, Vel, extending from 



pseuao-veium, ana never the ^.g^] lapp ets, ML, on either 



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 medusa? ; 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, Aureliaf) 

 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 



side; T, the small tentacles fringing 



