40 STRUCTURE AND PHYSIOLOGY 



protruded from this cell the body has a cylindrical figure, its 

 upper disk surrounded by eight short pectinated hollow tenta- 

 cula, in the centre of which the mouth is situated, leading into 

 a distinct stomach, which is as it were suspended in the centre, 

 and sustained there by eight thin membranous septa, which, 

 stretched between the outer surface of the stomach and the in- 

 ner surface of the external tunic, divide the intervening space 

 into eight equal compartments. The base of the stomach is 

 perforated like the mouth, and from the margin of the aperture 

 depend eight white tortuous filaments, which hang, either loose 

 or connected to a continuation of the membranous septa, in a 

 wide abdominal cavity, immediately underneath the stomach. 

 This cavity is again continuous with a tube which penetrates 

 the common mass, communicating freely bv anastomoses with 

 the tubes of other polypes, and with a fine net-work of capilla- 

 ry vessels, formed in the spaces between them, by means of 

 small apertures in their walls. ( Fig. 5. *) In this manner there 

 is effected a very free communication between the individuals 

 of each common mass, so much so, that the water swallowed by 

 iiny one polype of it rapidly permeates the whole, -f- By tra- 

 cing the course of the fluid we may obtain a clearer view of the 

 organization. The water then enters the mouth, and passes 

 through the cylindrical gullet and stomach into the abdominal 

 cavity ; thence part of it, flowing through the canals formed 

 by the septa stretched between the stomach and outer tunic, 

 passes into the tentacula with whose cavity the canals are con- 

 tinuous, and by means of small apertures in the sides of the 

 hollow tentacula, the water penetrates and unfolds the cilia, 

 with which these tentacula are fringed. By the distension from 

 the water thus introduced, the body of the polype and its ten- 

 tacula are forced beyond the surface, and every organ fully dis- 

 played. Another portion of the water in the abdominal cavity 

 passes into the tube continuous with it, fills it and the others 

 in connection with it, and by means of holes in their parietes 

 finds access into the intermediate capillary net-work, so that 

 the whole mass is permeated with the fluid, and all and every 



* A longitudinal section of Aleyoiiium tligitatum. 



-f- Milne- Edwards has proved this by a decisive experiment — Ann. dcs Sc. 

 Nat. iv. 3:28, and 338, an. 18.3.5. 



