HYDROIDEA. 329 



Embryology. Korotneff, Z. W. Z. xxxviii. 1883. (Transl. A. N. H. (5) xi. 1883.) 

 Artificial reversal of animal. Engelmann, Z. A. i. 1878 and Marshall, 'Vital 

 phenomena, &c., and new species of Hydra viridisj Z. W. Z. xxxvii. 1882. 



FIG. 7. Portion of tentacle of Eucopella campanularia; from Von Lendenfeld, Z. W. Z. 

 xxxviii. 1883, PI. xxviii. Fig. 4. 



THE tentacle, as in all Hydrozoa and Anthozoa, consists of ectoderm, 

 supporting lamina and endoderm. The ectoderm consists of three layers, 

 shown in A and B. 



A. The superficial stratum of covering cells, fully formed cnidoblasts and 

 sense cells. The covering cells have polygonal outlines, a superficial 

 delicate cuticle, a nucleus centrally placed, and protoplasm much 

 vacuolated, and disposed in a mass round the nucleus with irregular 

 peripheral strands, as seen in three of the more centrally placed cells. 

 These details are omitted in the remaining cells for the sake of 

 clearness, as also is the delicate cilium which each cell carries. Three 

 projections, two on the right and one on the left, are cnidocils (see 

 Fig. 8), and the nematocysts corresponding to them are seen as oval 

 bodies at their base. Both cnidocils and nematocysts are disposed at 

 an angle of 45 to the axis of the tentacle. At its tip, however, 

 they are placed at right angles. Sense -cells are not figured (see 

 Fig. 8, s'\ but they occur principally towards the apex of the tentacle, 

 scattered among the covering or supporting cells. 



B. Deeper layer of ectoderm, composed of ganglion cells, young cnido- 



blasts and muscle cells. A flattened ganglion cell, g y is figured with 

 its nucleus and outrunners. Von Lendenfeld did not observe any 

 anastomosis of these fibrils in Eucopella. There are ten ganglion cells 

 in the proximal portion of the tentacles in this hydroid, but none in 

 the distal. Between the ganglion cell and the edge of the covering 

 cell-layer is a young cnidoblast. The muscle cells, m, form a con- 

 tinuous layer applied to the supporting lamina, and this continuous 

 layer is absent only at the tip of the tentacle. They are here deeply 

 placed, i. e. sub-epithelially, as they are in some other Hydroids 

 (Plumularia, Eudendrimn) not superficially as in Hydra and others. 

 Each cell consists of a small quantity of protoplasm surrounding the 

 nucleus, and a long contractile fibre pointed at each end. The pointed 

 ends are shown turned up in the figure. There are many nuclei 

 scattered about, and the filaments are all disposed longitudinally, as is 

 the case in the tentacles of most Hydrozoa. 



C. The supporting lamina between the ectoderm and endoderm, and parts 

 of two endoderm cells. The lamina is very thin : not so the cell-wall 

 of the endoderm cells, which is thick and resistent, and shown in the 

 diagram as dark lines. The cells are arranged in a single linear series 



