124 
ME. H. N. MOSELEY ON THE 
In transverse hardened sections the body-cavity is seen to be entirely filled with the 
pigmented yellow cells, which also fill the canals of the hydrophyton. In the tentacles 
of the mouthless zooids, however, of which a glance was obtained under a high power, 
the transverse lines or apparent septa, so characteristic of the Hydroids (fig. 13), and 
considered by Allman to be in reality the opposed walls of large adjacent endodermal 
cells, were clearly seen, and also multiramified amoeboid-looking corpuscles occurring 
in the endoderm (RC, fig. 13), and resembling those figured by Allman as occurring 
in the tentacles of Coryne pusilla, and considered by him to be the nuclei of the large 
endodermal cells*. 
The body-cavities of the zooids were seen in the living condition to be filled with 
the yellow pigmented cells, and a few of these cells were seen occasionally to penetrate 
a short distance into the cavities of the tentacles, which cavities are continuous by 
widely open mouths with those of the bodies of the zooids. Giliation of the somatic 
cavities could not be seen. The spheroidal heads of the tentacles are composed of 
masses of closely-set thread-cells of various sizes and stages of development, but all of 
one peculiar kind (Plate 2. fig. 2), the larger ovoid thread-cells never occurring in them. 
A thin hyaline, apparently structureless ectodermal layer extends between these agglo- 
merated thread-cells, its marginal outline not being circular but depressed in short 
curves between the somewhat projecting tips of the cells (Plate 3. fig. 13). 
The mouthed zooids, when retracted, viewed directly from above, show the mouth in 
the centre, and four, five, or six tentacles arranged at equal distances around. The 
mouthless zooids, when retracted, have their tentacles closely drawn together, so as to 
form a hemispherical mass composed of the closely-set spheroidal tips of the tentacles 
(Plate 3. figs. 10 & 11). It can easily be understood how a vertical section through such 
a mass of retracted minute tentacles would give the appearance of a large compound 
tentacle, the small tentacles appearing to constitute the pinnse. I was misled by such 
a preparation ; and in my paper upon Heliopora ccerulea , lately presented to the Royal 
Societyfi, I stated my belief that the tentacles of Millepora would prove to be compound. 
The body-cavities of the zooids terminate interiorly in blind ends at the bottoms of 
the calicles, but are continued outwards at their bases in all directions into the canals 
of the hydrophyton, which join them all around, being disposed in an irregularly 
radiate manner (Plate 3. figs. 10 & 14). 
Muscular fibres, having a longitudinal disposition, are extremely well developed in the 
zooids. They arise for the most part in bundles from the radiating vessels of the hydro- 
phyton, which spring from the bases of the zooids, and pass up the w T alls of the bodies of 
the zooids, extending in the mouthed zooid nearly as far as the mouth. In the contracted 
zooids, when viewed directly from above or below, they have necessarily a radiate dispo- 
sition, as shown in fig. 14. Not all the fibres are gathered into the bundles; but some 
sparsely spread ones occupy the interval between these bundles, maintaining a like 
radiate course. The bundles may be traced for a considerable distance along the 
* Allman, ‘ Gymnoblastic and Tubularian Hydroids,’ pi. iv. fig. 3. + Phil. Trans, vol. 166. pt. 1. 
