COELENTERATA I93 



mesenteries cause a longitudinal retraction of the polyp, the transverse 

 muscles of the mesenteries in the neighbourhood of the stomodaeum 

 open the mouth when they contract, and the longitudinal muscles of 

 the tentacles when these are touched by particles of food contract so 

 that the tentacle bends towards the mouth and helps to push the food 

 inside it. The muscular system is for the most part under the control 

 of the nerve net. Although there is no central nervous system the 

 amount of contraction produced is proportional to the strength of 

 the stimulus. If a sea anemone is violently stimulated, e.g. touched by 

 a glass rod in any part the stimulus is transmitted to every muscle 

 and the whole animal shrinks to a shapeless lump. The process of 

 feeding is extremely complex and involves the action of the muscles, 

 the cilia and the glands. In a sea anemone like Metridium^ which lives 

 on the minute animals of the plankton, when these approach the oral 

 disc they are stunned by the nematocysts, snared by the mucus of 

 the glands of the tentacles, transported by cilia to the tips of the 

 tentacles, and pushed by the tentacles towards the mouth, which 

 gapes to receive them. Most remarkable of all, the cilia of the lips, 

 which normally maintain the outwardly flowing respiratory current, 

 reverse their beat to sweep the food into the enteron. While there 

 is this remarkable co-ordination of activities in feeding the nerve net 

 preserves the individuality in action of the parts so that the severed 

 tentacle of a sea anemone is able to execute movements just as if it 

 was still in place on the appropriate stimulation. In another common 

 anemone, Tealia, there are no cilia on the tentacles and oral disc, and 

 feeding takes place entirely by the muscular movement of the tentacles . 

 Sea anemones and corals are often nocturnal, remaining contracted 

 by day, expanding and feeding at night. In such corals as Lobo- 

 phy Ilium the tentacles are capable of enormous extension. In the 

 forms which feed by day like Fungia the tentacles are shorter and the 

 food is collected more by the action of cilia on the tentacles and oral 

 disc and less by the seizing of organisms by the arms and withdrawal 

 to the mouth. A remarkable biological feature is the frequent 

 presence of commensal algae (compare Hydra viridis) in the tissues 

 (Fig. 141 B, z.). This is especially the case in reef corals, in which 

 the most recent investigations show that the algae are of no nutritive 

 value while the oxygen they liberate in the tissues has no relation to 

 the needs of the coral. On the other hand the fact that they remove 

 excreta from the coral tissues is of importance. 



SuBPHYLUM CTENOPHORA 

 Free and solitary Coelenterata ; whose active locomotion takes place 

 by ciliary action ; which are not reducible either to the polyp or to the 

 medusoid type ; and are without nematocysts, but possess '* lasso cells ". 



