170 THE INVERTEBRATA 



of the mesenteries cause a longitudinal retraction of the polyp, the 

 transverse muscles of the mesenteries in the neighbourhood of the 

 stomodaeum open the moiith when they contract, and the longi- 

 tudinal 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. If a sea anemone is 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 action of 

 glands and cilia is not under the control of the nervous system, 

 but they work together with the muscles in the task of feeding. 

 In a sea anemone like Metrtdtum, 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. So while the nature of the nerve net 

 enables the severed tentacle of a sea anemone to execute movements 

 just as if it was still in place, there is this remarkable co-ordination of 

 activities in feeding. 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 Lohophyllium 

 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. This is 

 especially the case in reef corals, in which the most recent investiga- 

 tions show that the food value of the algae is practically nothing, while 

 on the other hand the fact that they remove excreta from the coral 

 tissues is of great 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". 



The Ctenophora, apart from certain aberrant forms, are globular, 



