58 EMBRYOLOGY 



into the first tentacles of the larva, while the middle part 

 becomes the body of the medusa (Fig. 23). The gastral 

 cavity is produced by the dissociation of the entoderm 

 cells, the mouth breaking through later. There is developed 

 a second pair of smaller tentacles, which with the first 

 pair forms a cross. By the development of the sensory 

 bodies, the mesogloea, the umbrellar cavity, and the velum, 

 the larva is gradually converted into the form of the 

 medusa (J. Müller, Metschnikoff). 



Although the development of the ^ginidse thus consists of a simple 

 metamorphosis, much more complicated relations have been found in 

 the life-history of the Cuninas, which are jDroduced by the parasitism of 

 the larva and the simultaneous tendency to early budding. ^ The con- 

 ditions in Cunoctantha octonaria are, according to McCrady and to 

 Brooks (No. 17), comparatively simiile. In this case the ciliate larvffi get 

 into the umbrellar cavity of one of the Tiaridaj (Turritopsis), and there, 

 through stages similar to those described above for iEginopsis, grow up 

 into an actinula-like creature, which attaches itself by means of its four 

 tentacles to the outer wall of the stomach of Turritopsis, while it intro- 

 duces its long proboscis through the mouth-opening into the stomach of 

 its host. This larval stage multiplies by budding until finally both the 

 original larva and the individuals thus produced acquire the form of 

 medusffi by a gradual metamoi-phosis, and become young Cunoctantha^. 

 Similar are the cases in which free-swimming planulaj of Cuninas mi- 

 grate into the stomach of Geryonidae and there attach themselves, and 

 grow up into a spike of buds IKnospenäln-e], Since in these cases, how- 

 ever, the buds alone possess the capability of being metamorphosed into 

 medusffi, whereas the polypoid stolon developed out of the larva does not 

 undergo further development, the outcome is the establishment of an 

 alternation of generations. There have often been observed in the gas- 

 tral cavity of Cuninas themselves parasitic larv£e of Cuninas, which 

 became metamorphosed into medusae, but at the same time multiplied 

 asexually by budding at the aboral joole (Metschnikoff). Since the in- 

 dividuals thus produced often differ essentially in structure, especially 

 in the number of the antimera, from the forms in whose stomachs they 

 are found, it has remained doubtful whether one had to do in this case 

 with a brood differing from the parent in form or with descendants of 

 another species of Cuniua, which in the free-swimming stage migrate 

 into the gastral cavity of the host. Eecently a Cunina larva (?) para- 

 sitizing the mantle-jelly of Salpa fusiformis has been described by 

 KoROTNEFF (No. 36) as Gastrodes parasiticum.* 



* [Compare 0. Maas, No. X., Appendix to Literature on Hydroidea.] 

 2 [In regarc^to the position of Gastrodes, compare Korotneff, No. 

 VIII., and Heid'er, No. VII., Appendix to Literature on Hydroidea.] 



