MEiM. M. C. Z., IX. No. 3.— ACALEl'HS, POLYPS. 30 



11. Tlie same as fig. 10, speii in inotile. The cesoiJiagus (phalangeal sac) descending from the mouth is already 



well formed, and the chambers to the number of four are also well limited. 



12. An embryo mueh larger than the preceding. The small compartment (a) is divided into three by the pair 



of partitions, 3. The compartment /3 of fig. 10 is also divided alreaily into two (/3 and 6) by the appear- 

 .ance of the partition, 4. Of the S septa thus acquired, the pairs numbered 1 and 2 are always more de- 

 veloped than the others. 



Note. — The order of development of tlie "septa" 2 and 4 is claimed by the brothers Hertwi'^ to 



have been interchanged, so that the true order of their appearance would be indicated by tlie numbers 



in parentheses. Compare explanations of ngs. 31, 35, and 36. 



13. Profile view of tlie preceding, showing the mouth surrounded by 8 compartments with rounded bases. 



14. Th(^ same as in the two preceding figures, but the progress of the partitions being more considerable, the 



division into 8 compartments is more distinct ; the partitions, 1, are still much more advanced ; they 

 already reach the centi-al pad surrounding the mouth (peristome), and consecpiently begin to indicate the 

 primitive division into halves. 



15. An embryo in which the 8 complete partitions have reached the peristome. 



16. Embryo with 8 divisions showing the partitions, 5, well advanceil, and the beginning of the jiartitions, 6, in 



the lobe 7. The two new chambers thus formed are respectively e ami j". 



17. One of the various forms which the embryos assume when in motion. 



18. The same as fig. 17. It has been compressed a little to show the "septa" or mesenterial folds (mnl.). 



Only two of the latter (1, 1) present the beginnings of the craspeda, or mesenterial filaments ; they are the 

 primary folds. 1^. 



19. An embryo represented with the form whicli it assumes when it swims rajiidly. The tuft of cilia at the 



pedal pole is considerably elongated. '^. 



20. Profile view of an embryo. The mouth is sup[iortcd on a snout-like prolongation, and the primitive cham- 



ber a' has begun to send forth a tentacle. 1, Piimary fold with mesenterial fihiment slightly developed.'/. 



21. Embryo further developed than the preceding, exhibiting already eight tubercles, which are the beginnings 



of the tentacles of the eight chambers first formed, 'r. 



22. Oral aspect of a young actinia already approaching the form of the adult. The 12 tentacles of the first for- 



mation are already jiroduced. The period of eciualization in sixes, taken alternately, is in jirocess of 

 accomplishment and transforms the special embryonic form into one with regularly radial symmetr\'. The 

 two cycles already begin to appear. The group of 7 lobes which has as its centre the tentacle a' is always 

 distinguishable Ijy the size of the latter, and by the development of the mesenterial folds, 1. 'i''. 



23. Young actinia with two well-pronounced cycles. The limits of the pedal disk are well marked, although the 



movements of the animal are still very lively, and the tuft of pedal cilia is very long. -j". 



24. Young actinia already attached, viewed from the oral pole. In the intervals between the tentacles of the first 



cycle (a — f, f — e (f — 5?), S — a') there begin to be for-med six new pairs of young tentacles, which in- 

 crease the total number to twenty-four. 



Note. — Of the three tentacles occupying each of the intervals (a — f, f — 5, etc.) the middle one 

 out-strips the other two, replacing (in size) 7, e, /3, etc., and thus the middle ones come to constitute 

 the second cyde. The third cycle is then composed of the remaining 12 (smallest) tentacles, which 

 occupy the intervals between those of the first two cycles, thus regularly alternating with them. The 

 subsequent increase in the number of chambers — and later, in that of the corresponding tentacles — 

 is accomplished by the production of a pair of elements (mesenteries) in each of the 12 chambers above 

 which are placed the smallest tentacles ; and by a process of substitution similar to that just described 

 the middle ones of the three compartments (tentacles) thus formed comes to constitute the tliii-d cycle, 

 .while the two on either side of it become menibers of the fourth cycle. The fifth cycle is formed in a 

 similar manner. " Since after the formation of each frebh cycle, the arrangement of the tentacles 

 again becomes symmetrical (in sixes), it is obvious that all the equal-sized cycles except the first are 

 formed of tentacles entirely heterogeneous as to age." 



25. Face view — from the side of the seven-chambered group — of a young actinia with twenty-four tentacles, 



showing the relative ilevelopment of the three pairs of mesenterial folds, 2, 4, and 5. '1'^. 



26-31. Actinia (sp. ?). From A. 0. Kowalevsky, Observations on the development of the Calenterata. From 

 the Publications of the Imperial Society of Friends of Natural Sciences, Anthropology, and Ethnography. 

 Moscow: Katkov, 1873. 4to, 36 pp., 8 pi. (Russian.) PI. IV. figs. 1-5, 7. 



26. The egg after its segmentation. 



27. Infolding of the blastoderni. 



28. Inv.aginatiou completed. 



29. Later stage in which the mouth-opening appears as a narrow slit and the first pair of mesenterial partitions 



has arisen. 



30. Kadial section of the germ which is represented in fig. 29. 



