THE ANATOMY OF THE MADREPORARIA. 
581 
that of Alcyonium digitatum ; the polyps live in small calyces 
on the surface of the colony, but the living tissues are not 
continued down into its centre, as in Alcyonium ; the lower 
part of the cavity formerly inhabited by the polyp being 
shut off by a kind of tabula as it grows upwards. Over the 
surface of the colony lies the coenosarc, the fleshy rind of the 
otherwise calcareous colony, which puts the polyps in commu- 
nication with one another, being permeated by canals which 
are continuous with their coelentera, and similarly lined 
by endoderm. The polyp possesses six pairs of mesenteries, 
six larger tentacles, six smaller tentacles, and six entosepta. 
There are two distinct types of nematocyst. Longitudinal 
muscles occur on the mesenteries, but the smallness of the 
latter rendered it impossible to detect whether their arrange- 
ment agreed with Actinia or not. 
In 1881, Dr. von Heider, of Graz, published a description 
of Cladocora astraearia and Cl. cespitosa (8). These 
species are also built on the Actinian type ; and Heider 
describes for them the same continuation of the mesenteries 
and mesenterial spaces that v. Koch mentions as occurring in 
Caryophyllia cyathus. I have examined macroscopically 
and by sections Cl. cespitosa in a completely retracted state, 
and can find no trace of such a condition, an observation which 
confirms my belief that this appearance is due to partial contrac- 
tion, owing to the use of alcohol. There is no true coenosarc 
such as occurs in Stvlophora; just as there is no true coenen- 
chyme, the calyces being free outwardly from the rest of the 
colony. In luxuriant growth and budding, however, according 
to Heider, both skeletons and soft tissues of adjacent polyps 
may fuse ; an observation interesting as probably indicating 
the history of the formation of the coenosarc and coenenchyme 
which characterise many other forms. There is one correction 
to be made in his work, which for the sake of future workers 
in this field ought to be mentioned here ; namely, that in his 
PI. III. he frequently figures as endodermal cells small spherical 
bodies with a well-staining nucleus, which are zooxanthellae or 
symbiotic unicellular Algse, living free in the coelenteron in 
\OE, XXV. NEW SEK. 
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