ZOOLOGY AND BOTANY, MICEOSCOPY, ETC. 853 



membranes formed of cellulose, but with a special form of mesodermal 

 cell. 



After describing in great detail the various organs, and giving the 

 name of ovariostomata to endodermal passages wbicb seem to form a 

 kind of passage for the larvae, the author comes to some considerations 

 on the genetic relation of the Acraspeda to Corals. 



Observations on Xenia fuscescens showed that (1) the margin of 

 the mouth-disk and the pinnate tentacles execute rhythmical move- 

 ments, and regularly approach and separate from one another. These 

 contractions remind us of what may be seen in the umbrellas of 

 Medusae. (2) The rhythms of the two are almost identical, being 40 

 per minute in Aurelia and 30 in Xenia. (3) The contractions con- 

 tinue if the tentacles be separated at their base from the body-wall. 

 (4) Longitudinal sections of individual polyps leave the two halves 

 still capable of contraction. On the whole, then, there seem to be 

 striking resemblances in the nerve-physiology of Acraspeda and 

 Anthozoa. 



In conclusion, it is suggested that the Anthozoa have possibly had 

 a polyphyletic origin, and we are reminded of Semper's observations 

 on the alternations of generations in Fungia. 



Development of Obelia.* — C. de Merejkowsky has studied the 

 mode of development of the endoderm of this Medusa, and finds that 

 it is formed at the posterior end of the larva by the immigration of 

 isolated blastodermic cells. This mode of formation has already been 

 detected by 0. Schmidt in the sponge Ascetta, and Metschnikoff, who 

 has confirmed those observations, has extended them to Halisarca. 

 Kerschner, likewise, has observed the same phenomenon in the fresh- 

 water Hydra, and Metschnikoff and Glaus have seen it in various 

 Medusae. In higher Metazoa the same process has not been detected, 

 but in Echinoderms, Selenka has observed that the mesoderm arises 

 from immigrated cells ; while Kowalevsky, though suspecting, was not 

 able to demonstrate the process in Thecidium ; and Balfour has ex- 

 pressed the opinion that there was some error in the observation. 



The first question which arises is this : Is the mode of formation 

 seen in Ohelia and other Medusae a special process, having no relation 

 to the ordinary gastrula-condition, where the endoderm is formed by 

 invagination from a part of the blastoderm ? The author thinks not, 

 and, in support of his view, points to the fact that the cells arise from 

 that part of the larva which, in other Coelenterates, undergoes the 

 regular invagination. 



If we compare Ohelia with such a form as Pelagia, we may say 

 that in one the cells of the pole immigrate all together, without losing 

 their relations to one another, while in Obelia they become detached, 

 and immigrate separately. The slightness of the difference may be 

 demonstrated by a consideration of what sometimes happens in an 

 allied genus {Irene pellucida f) ; here, as a rule, the endoderm is 

 formed by separate cells, as in Ohelia, but it occasionally happens 

 that a typical gastrula is developed. 



* Bull. Soc. Zool. France, viii. (1883) pp. 98-129 (2 pis.).. 



