22 Coelenterata. 



the dorsal directive is complete in that it reaches the stomodaeum, but it is 

 not provided with a filament on its free edge. In sladeni the mesenteries bear 

 outgrowths, circular in section and each consisting of a mesogloeal axis sur- 

 rounded by vacuolated endoderm, in which gland-cells and nematocysts are not 

 present. These outgrowths fuse together near the lower end of the stomodaeum. 

 In gardineri there are, associated with the complete mesenteries, structures 

 which appear to represent stages in the down growth of stomodaeal ectoderm to 

 form mesenterial filaments. A larva, referred to Dactylactis, n. sp., has 11 mar- 

 ginal and 10 oral tentacles, and 12 mesenteries (5 on each side of the 

 2 directives). The ectodermic longitudinal muscles are well developed, but 

 there are no mesoglreal ridges supporting them. The entoderm is much vacu- 

 olated, and there are no acontia. In 2 larvae, referred to n. spp. of Ovaetis, 

 longitudinal muscle-fibres are not present in the lateral mesenteries, and there 

 are no acontia; both larvae have 8 mesenteries on one side and 7 on the other 

 side of the 2 directives; there are filaments on all the mesenteries except 8,8, 

 and 7 left. One of the larvae has no tentacles, in the other these organs are 

 just appearing. There are also described 6 Hexactinian larvae, with 8, 12 and 

 20 mesenteries, but without tentacles. Within the endoderm of one of the 

 larvae were found 3 embryos in different stages of development, and in all parts 

 of the endoderm there are cells apparently reproductive with large nuclei. 

 For Zoanthella see, infra p 29, Gary. 



Bohn has observed that Actinia becomes progressively insensitive when it is 

 exposed to light. Further, the various reactions are not the same by day as 

 by night, after being in the light and after a period in the dark. The light 

 seems to destroy certain chemical substances (? oxydases) in the living tissues. 

 Repeated excitations, or prolonged activity cause insensitiveness, no doubt owing 

 to loss of active substances. The sensitiveness of organisms may be augmented 

 by chemical, physical or mechanical means. 



Kinoshita finds that in repeated stimuli Adamsia, Bunodes, Heliactis and 

 Eudendrium, the duration and strenght of the contraction after single stimuli 

 diminuish with the number of stimuli, and that each stimulus exercises a less 

 effect than the preceding. The Actiniae in general do not react as often as 

 Tunicata to artificial stimuli, even the 3d or 4th stimulus being often ineffective. 

 The reaction is greatest to mechanical and electrical stimuli. Various poisons 

 destroyed the power of response to stimuli, but after removal of the poison the 

 power was recovered. 



Parker notes that a few seconds after a mechanical or chemical stimulus has 

 been applied to the ectoderm of the lower part of the column of Metridium 

 marginatum, the anemone contracts the longitudinal endodermic muscles of the 

 mesenteries, so that the oral disc becomes retracted. The fibrillar layer 

 in the base of the ectoderm and endoderm, believed by the Hertwig's to be 

 nervous, does not offer an easy means of explaining this reaction, since it does 

 not directly connect the ectoderm with the mesenteric muscles. By means of 

 a silver method, a rich system of neuro fibrils can be demonstrated in the 

 mesogloea. These fibrils penetrate the ectoderm and also extend to the longi- 

 tudinal mesenteric muscles; they are the essential nervous elements in the 

 retraction reflex. When a sea-anemone is cut nearly in two, nervous trans- 

 mission from one piece to the other can be accomplished through almost any 

 part of the body, except the lips. Yet in these the so-called nervous layer of 

 the Hertwig's is as well developed as in any other part and, therefore, is pro- 

 bably not the true nervous system. It is composed of the fine basal branches 

 of the epithelial cells, and may be a mechanism for the absorption of nourishment 



