220 THE BIOLOGY OF THE SEA-SHORE 



independent effectors and their activity is due to direct 

 stimulation from external sources. It is interesting to 

 note, however, that an injury performed within a centi- 

 metre of the osculum will induce the latter to close, so that a 

 method of transmitting an impulse is not entirely absent 

 in sponges. Moreover, in some cases at least, the transmis- 

 sion of the stimulus occurs without visible motion and 

 would therefore appear to be nerve-like (Parker, op. cit.). 



Sea-Anemones. — The body of an anemone is composed 

 of two organised cell-layers and a third non-cellular layer 

 or lamella placed between them which discharges the 

 function of a skeleton. The outer and inner layers (ecto- 

 derm and endoderm) have, notwithstanding the different 

 functions they discharge, very much the same structure. 



In each the epithelial outer portion contains sensory 

 cells whose deep ends are produced into a number of delicate 

 fibres interlacing to form a middle nervous layer which 

 includes, in addition, the so-called " ganglion " cells 

 already mentioned. Beneath this is a third layer, consisting 

 almost entirely of elongated muscle cells with their bases 

 closely applied to, or even embedded in, the supporting 

 lamella. The nervous organisation of the anemone thus 

 includes, in addition to effector and receptor elements, 

 the beginning of an adjustor element in the shape of a nerve- 

 net which, as we have already seen, permits a stimulus 

 affecting only a small sensory area to be conveyed to a 

 comparatively large system of effectors. It is only fair to 

 point out, however, that doubts have recently been raised 

 as to the importance of the nervous sub-layers in the ectoderm 

 and endoderm in nervous conduction, and it is now considered 

 that a more important part in the transmission of impulses 

 is played by the lamella, which has been shown to contain a 

 quantity of nervous elements and connections (Parker, o/). a*/.). 



Generally speaking, the responses of Coelentera are 

 distinctly of a quick type, though slower than those of 

 Arthropods, and this is due to the possession of sensory 

 surfaces, since by means of these the muscles are called into 

 action more quickly than by direct stimulation. 



