Form of Freshwater Ccelenterate. 117 



consists of the ectoderm and muscular layer, while the endo- 

 derm forms the wall of the paired cavities, which extend into 

 the tentacles and communicate with the surrounding medium 

 through the buccal aperture, which is situated in the middle 

 of the upper pai't of the mobile protrusible proboscis. 



As the development of the Polypodium does not conclude 

 with the form above described, we may, with reference to its 

 relations to the following, likewise free-living, generation, 

 designate it as the " mother form " (B). The mode ot 

 increase, however, still (and apparently for a long time) con- 

 tinues to be asexual, and when it becomes sexual subsequent 

 investigations will probably show, as I have not succeeded, as 

 already stated, in discovering the sexually mature form. 



Under normal conditions the mother-form (B) provided 

 with twenty-four tentacles (figs. 7 and 8) produces two 

 daughters by regular division, i. e, halving (B^, figs. 9-11), 

 each with twelve tentacles. These then divide further, each 

 giving origin to two different grandchild-forms (B'^ and Why 

 figs. 12, 13), each of which has six tentacles, which in both 

 forms of the last generation attain different lengths *. 



This is the ordinary course of increase ; but, as subsequent 

 observations have shown me, it does not remain stationary at 

 the grandchild-form ; but not only the two grandchild-forms, 

 but also the daughter-generations, again reproduce the motiier- 

 form or grandmother-form (B) by rapid growth and new 

 formation of the deficient number of regularly arranged 

 tentacles (figs. 14, 15). This is evidently not sufficient to 

 produce the numerous progeny of the primary form (B), for 

 by the cultivation of the free-living animals 1 have collected 

 indubitable facts which prove that the second form with 

 twenty-four tentacles, originating by the new-formation of 

 tentacles just mentioned (which I will designate «Bj, divides 

 again and produces a second daughter-genei'ation (aB^) and 

 a second grandchild-generation (aB" and aB^6). It is remark- 

 able, however, that the last generation again consists of two 

 forms, one of which is smaller and has shorter tentacles than 

 the other. 



As each of the thirty-two buds {h) of the common stolo (A) 

 divides further as the free-living form (B), we obtain as the 

 final result of this division from each stolo about five hundred 

 great-grandchild forms (aB^ and a Wh) . 



In comparison with the slow process of development of the 



* It may be remarked here that the best means of fixing the tentacles, 

 which react to the slightest irritation, is the cocaine solution quite recently 

 recommended by Prof. Richard (Zool. Anzeiger, no. 19(3). 13y means of 

 this we obtain preparations very true to nature. 



