ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 591 



be followed from T. madra of the Upper [Lias to T. cylindraceus of 

 present times. " The most remarkable feature in such ThecocyatM as 

 T. mactra and nathorsti is the great change to which they have been 

 subject ; at an earlier stage resembling a Deltocyatlius or Brachyeyathus, 

 and then assuming the calicular form characteristic of the genus. It 

 may be that this is a phenomenon indicating the ancestry of these 

 corals." 



Stinging-cells in Siphonophora. * — Karl Camillo Schneider has 

 made a very detailed study of the origin, development, and phenomena 

 of discharge of these structures. His more important results may be 

 concisely stated as follows : — The stinging-cells or cnidocytes contain 

 a central cnidum and an apical discharge apparatus. The cnidum con- 

 sists of a capsule or reservoir of poison, and a tube or injection 

 apparatus. By means of the discharge apparatus the cell-operculum 

 springs back, and allows the water to come into contact with the 

 stinging-secretion, which takes up water and swells suddenly. The 

 stinging-secretion is morphologically the most important part of the 

 cells, which are to be regarded as modified gland-cells (Lendenfeld). 

 The cnidocyte has also certain accessory structures which attach it to 

 the epithelial cells. The cuidum has a complicated life-history, which 

 may be studied under the following headings : — (1) A growth period, 

 lasting until the tube is invaginated into the capsule ; (2) the invagina- 

 tion period ; (3) the preliminary maturation period ; (4) the wandering 

 period, during which the organs reach the spot where they are used later ; 

 (5) the final maturation period. A resting period then follows, which 

 is closed by the instantaneous discharge of the cells. The structure of 

 the various parts during the different stages, and the mechanism of dis- 

 charge, are described in much detail. 



Development of Aurelia.t — Dr. W. Hein has re-investigated this 

 much-disputed development, and sums up his observations as follows. 

 The blastula is composed of similar cells throughout, and has a central 

 blastocoele. Into this cavity certain cells wander from the blastoderm, 

 and undergo speedy degeneration. Gastrulation occurs by a typical 

 process of invagination, but the blastopore is small, and persists as a 

 fine canal until the formation of the mouth. Rarely certain endoderm- 

 cells wander into the archenteron and there undergo degeneration : but 

 there is no evidence that the cells which wandered earlier into the seg- 

 mentation cavity take any part in the formation of the endoderm. While 

 the rounded gastrula is gradually becoming transformed into the oval 

 planula, the oral endoderm, by its more rapid division and the small size 

 of the cells, becomes differentiated from the atrial endoderm. After 

 fixation, this mass of proliferating oral cells increases in size, and the 

 prostoma is converted into the definitive mouth ; gullet-tnbe and gastric 

 pockets were not observed. The four primary mouth-tentacles originate 

 simultaneously, soon after the formation of the mouth. At this stage 

 four small interradial ingrowths arise from the upper part of the endo- 

 derm, grow like folds into the archenteron, and form the gastric ridges. 

 From the manner of their origin four perradial grooves arise between 



* Arbeit. Zool. Inst. Wien, xii. (1900) pp. 133-242 (7 pis.). 



f Zeitschr. wisa. Zool., lxvii. (1900) pp. 401-38 (2 pis. and 4 figs.). 



