STKUCTUKE OF A SPECIES OF MILLEPOEA. 
123 
the upper extremity. They have no trace of a mouth, nor of any of the gastric cells 
of the mouthed zooids in their body-cavity. They bear tentacles at irregular intervals 
from near the bases to the summits of their bodies. The tentacles are very variable in 
number; some zooids have only five tentacles, whilst all numbers from 5 to 20 
(and possibly, in exceptional cases, a slightly greater number) occur in others. From 
12 to 15 is the most usual number. The tentacles consist of a cylindrical stem, longer 
and more slender than in the mouthed polyps, and a spheroidal tip resembling that of 
the tentacle of the mouthed zooid, but smaller. The body of the zooid terminates 
sometimes in two, sometimes in three tentacles, springing from a common point. 
The mouthless zooids expand far more readily and quickly than the mouthed zooids, 
of which latter it is comparatively difficult to obtain a view in the expanded condition. 
The short mouthed zooids appear to remain perfectly quiescent when expanded, whilst 
the mouthless zooids are in constant serpentine motion. The mouthless zooids carry 
their bodies seldom extended straight, but usually bent in several curves ; they appear 
to bend over towards their mouthed zooid from time to time, as if to convey food. All 
the zooids are retracted on alarm with remarkable suddenness, disappearing entirely 
within the calicles. 
When a portion of the coral has been placed living in reagents, it is found, when 
hardened, to be bristling all over with sheaves of threads shot from the thread-cells 
around the mouths of the calicles. By some accident, on one small portion of a coral 
placed in absolute alcohol, the mouthless zooids all remained partially protruded. This 
was only over a small area of about ^ of a square inch in dimensions, enough to yield a 
single microscopical preparation. From a very large quantity of the coral prepared in 
an exactly similar manner, no second preparation could be obtained, though it was all 
searched over carefully for similarly expanded zooids. This fact, however, shows that 
perhaps it might have been possible to obtain a larger quantity of expanded zooids in 
the hardened condition by the gradual addition of alcohol or fresh water to the sea- 
water in which the living animals were expanded, or by some similar means ; or 
perhaps by sudden addition of osmic-acid solution as recommended by F. E. Schulze*. 
The body of the zooids, when seen in transverse section, is found to consist (fig. 15) 
of an ectodermal layer, beneath which is a layer of membrane, and an internal mass of 
endodermal cells. The ectodermal layer, as studied in sections of hardened specimens, 
appears to consist of well-defined cells, most of which contain small thread-cells, whilst 
some contain simple nuclei. The membranous layer is apparently structureless ; it 
extends throughout the body and tentacles. Beneath the membranous layer, and in 
close union with it, are the muscular structures to be presently described, and within 
these, in the case of the mouthed zooids, are, in the upper region of the body, the 
gastric cells already described. The structure of the endoderm in the lower part of the 
body of the mouthed zooids, and in the mouthless zooids, was not well ascertained. 
* Anleitung zu wissenschaftlichen Beobachtungen auf Eeisen. Herausgegeben von C. Hetjmayeb, Hydrograph 
der kaiserlichen Admiralitat. Berlin, 1875. Wirbellose Seethiere von K. Mobius, p. 424. 
