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 
any one polype of it rapidly permeates the whole. 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 Alcyonium digitatum. 
Milne- Edwards has proved this hy a decisive experiment — Ann. des Sc. 
Nat. iv. 328, and 338, an. 1835. 
