454 SUMMARY OF CURKENT RESEARCHES RELATING TO 



different parts of one and the same colony to an extent which, if 

 noticed in separate colonies, would be regarded as affording grounds 

 for generic distinction. 



New fresh-water Coelenterate— Microhydra Ryderi.* — An inter- 

 esting fresh-water polyp-form has been discovered near Philadelphia 

 by Mr. E, Potts, and described by Mr. J. A. Eyder. Simpler and much 

 smaller than Hydra, this Microhydra Ryderi exactly resembles a fixed 

 planula, without cilia and provided with a mouth. The latter is small 

 and exhibits an irregular opening ; there is no disc-like expansion, 

 nor hint of tentacles. The nematocysts of the thin ectoderm are 

 mostly near the mouth. An indistinct layer beneath the ectoderm 

 probably represents contractile processes of the outer cells. Eound 

 the mouth the endoderm consists of solid cells ; below this narrow 

 zone, for the upper third of the polyp, the endoderm consists of large 

 cells, with distinct vacuoles and small excentric nuclei. 



Sexual reproduction has not been observed ; but asexual budding 

 has been repeatedly studied through several generations. The 

 Microhydra-hnd., however, unlike that of Hydra, is formed longitu- 

 dinally ; the parent and the bud lie side by side. When separation 

 occurs the bud falls to the foot, remains for a while motionless, then 

 fixes itself at the aboral pole, and begins an independent life. 

 Artificial division has not yet been tried. If the above account be 

 correct the simplest Coelenterate is certainly Microhydra. 



New Zoanthese.t — Dr. A, Erdmann commences with some ob- 

 servations on the characters of the septa, which he distinguishes as 

 dorsal and ventral ; in some the dorsal septa which are directed 

 towards the ventral zone consists only of macrosepts, and this type 

 may be distinguished as the macrotype from the more common micro- 

 type. In all other Actiniae (excepting the Cerianthidas) every pair of 

 septa is capable of producing new pairs of septa, but in the Zoanthesa 

 only two interseptal processes are capable of producing new septa ; 

 they are the two which lie nearest the ventral directive septa, Zoan- 

 these are either free-living or colonial, and colonies are either formed 

 by delicate branching stolons given off from the base, or they are 

 placed on a more or less extended coenenchym, or, lastly, the polyps 

 are sunk into a common coenenchym. This last is always traversed 

 by connecting tubes lined by endoderm, and continued directly into 

 the interior of the polyps ; the mesoderm has a number of special 

 cavities filled with cells, and the canals and cavities appear to be 

 always of ectodermal origin ; numerous rounded or stellate connective- 

 tissue bodies are also to be found in the mesoderm ; the mesodermal 

 filaments have their lower halves formed by an unpaired glandular 

 streak ; in the middle there is a paired ciliated streak ; a table of the 

 five known genera is given with the distinctive marks, drawn from the 

 characters of the septa, the circular muscle, the coenenchym, integu- 

 ment, and disposition of gonads. Of the thirteen forms whose de- 

 scriptions follow, Polythoa axinellse is alone referred to its species, the 



* Amer. Natural., xix. (1885) pp. 1232-6. 



t Jenaiach, Zeitschr. f. Natiuwiss., xix. (1885) pp. 430-88 (2 pis.). 



