162 PHYLUM C@LENTERA. 
ridges of reproductive cells, almost always either ova or 
spermatozoa, rarely both; and (d) in some cases offensive 
threads or acontia. The mesenteric filaments seem to be 
closely applied to the food, and perhaps secrete digestive 
‘juice. Intracellular digestion also occurs. Sea-anemones 
have no sense organs; the sapphire beads, which are so 
well seen at the bases of the outermost tentacles of the 
common Actinia mesembryanthemum, are batteries of 
stinging cells. The nervous system is uncentralised, and 
consists of superficial sen- 
sory cells connected with a 
plexus of sub- epithelial 
ganglion cells. 
The layers of the body.— 
The ectoderm which clothes the 
‘ exterior is continued down the 
inside of the gullet. The endo- 
derm lines the whole of the 
internal cavity, including mes- 
enteries and tentacles. The 
mesogloea is a supporting plate 
between these two layers, and 
forms a basis for their cells. 
The ectoderm consists of 
ciliated, sensory, stinging, and 
glandular cells, and also of sub- 
epithelial muscle and ganglion 
Fic. 
80. — Section 
anemone (across arrow in Figure 
79).—After Andres. 
A, B, Directive septa; mf, mesenteric 
through _ sea- 
filaments; g., genital organs; 7.., 
longitudinal muscles; s., primary sep- 
tum ; s’:, secondary septum ; s”., tertiary 
septum. The arrow enters between two 
primary septa (an intra-septal cavity), 
and passes out between two tertiary 
cells based on the mesogloea, but 
mainly restricted to the circum- 
oral region. 
The endoderm consists mainly 
of flagellate cells, with muscle 
septa. fibres at their roots. These form 
the chief muscle bands of the 
wall, the mesenteries, and the gullet. Nor are glandular and even 
sensory cells wanting in the endoderm. 
The mesenteries.—In sea-anemones and nearly related Anthozoa, 
twelve primary mesenteries are first formed. These are grouped in 
pairs, and the cavity between the members of a pair is called intra- 
septal, in contrast to the inter-septal cavities between adjacent pairs. 
In these inter-septal chambers other mesenteries afterwards appeat in 
pairs. Two pairs of mesenteries, however, differ from all the rest—those, 
namely, which are attached to.the two corners of the mouth and to the 
corresponding grooves of the gullet. These two pairs of mesenteries 
are called ‘‘ directive,” and they divide the animal into bilaterally sym- 
metrica ‘halves. Anatomically, a pair of directive mesenteries differs 
from the other paired mesenteries, because the retractor muscles, which 
