98 KECORD OF CURRENT RESEARCHES RELATING TO 



tliree layers — an outer mucous layer, a middle thin, homogeneous 

 layer, divisible into arese corresponding with the cells, and an inner 

 much thicker layer, composed of superposed polygonal plates corre- 

 sponding with the cells, each plate being again made up of thin, 

 highly refracting rods. 



The ectoderm is not pigmented, and contains numerous nemato- 

 cysts, mostly sausage-shaped, a few pear-shaped. In certain parts 

 are batteries of nematocysts, consisting of a globular ingrowth of 

 ectoderm, the outer layer of cells of which form an investment, while 

 the greater number of the included cells produce nematocysts. When 

 the animal is strongly irritated, the whole battery is discharged, cells 

 as well as nematocysts being extruded. 



2. Mesoderm. — This layer consists of a ground-substance of gela- 

 tinous character, containing elastic fibres, and contractile elements of 

 two kinds. The first of these, forming the musculature of the gastric 

 region, tentacles, &c., consists of mere muscle-processes of the ectoderm 

 cells, like those described by Kleinenberg in Hydra ; the ectoderm 

 cells tapering at their inner ends, and being continued into the con- 

 tractile fibres. The eight circular and longitudinal muscles consist 

 of true fusiform muscle-cells, with distinct nucleoli ; they may attain 

 a diameter of 4 fx, and a length of as much as 5-10 mm. 



3. Endoderm. — The cells of this layer are flagellate, and contain oval 

 nematocysts of variable size. Amongst them are special gland-cells ; 

 probably both these and the flagellate cells secrete a digestive fluid. 



Kliug assigns an important digestive function to the filaments in 

 the gastric cavity, which exhibit during life a slow vermicular move- 

 ment. They are plano-convex in section, and consist of a hyaline 

 axis surrounded by endoderm cells, of which those on the plane 

 side are exclusively glandular, while those on the convex side are 

 flagellate, and contain, especially towards the free end of the filament, 

 nematocysts. 



4. Tentacles. — The endoderm cells of the stalks of the tentacles 

 are very large and elastic, 1ind seem to act as antagonists to the mus- 

 cular fibres which produce retraction. 



The ectoderm of the knob terminating each tentacle has a very 

 complicated structure ; the isolated cells look not at all unlike retinal 

 elements. At the outer or distal end of each cell is a thickish cuti- 

 cular layer ; then comes an enlargement bearing a sausage-shaped 

 nematocyst, with a palpocil or cnidocil projecting through the cuticle ; 

 beyond the nematocyst the cell is filamentous, having, however, an 

 enlargement near the middle containing the nucleus, and another 

 smaller enlargement near the proximal end, where the cell usually 

 ends in a slight swelling, from which two or three processes are 

 given off. Owing to this complicated structure, the whole ectoderm 

 of the knob has the appearance, in section, of being divided into a 

 number of layers. 



5. Marginal Papillce. — These are, when present, usually eight 

 in number, four being radial and four interradial. In the majority 

 of individuals, however, they are absent, and in some they are fewer 

 than eight. In structure they resemble undeveloped tentacles. 



