372 REPORT—1863. 
peculiar receptacle, where they undergo further development, and which is sup- 
ported upon the summit of the gonangium, and entirely external to its cavity 
(fig.99,h). It will be found convenient to employ a special term for this 
receptacle, which confers upon the gonosomes in which it occurs a very cha- 
racteristic feature. I have already designated it by the name of “ acrocyst” *. 
It may be seen in Sertularia pumila, S. cupressina, S. polyzonas, Calycella 
syringa, &c., and would seem to be in every instance confined to the female. 
There is some difficulty in determining the exact morphology of the 
acrocyst. In its usual form it seems to consist of a simple extension of the 
endotheca of the gonophore, protruded as a hernia-like sac through the 
summit of the gonangium, while the whole becomes surrounded by a thick 
gelatinous-looking envelope, which is excreted from the outer surface of the 
sac, and which shows no appearance of true structure, though distinct zones 
of deposition may occasionally be observed in it. 
In Calycella (Laomedea) lacerata, Johnst., the spadix itself, as has been 
correctly stated by Dr. 8S. Wright, is, with the surrounding endotheca and 
ova, carried upwards upon the blastostyle, by whose elongation it is protruded 
as an acrocyst from the summit of the gonangium, when the whole becomes 
invested by the usual thick gelatinous excretion. The peculiarity of the 
acrocyst in this case is found in the presence within it of the spadix, which, 
however, is depressed by the enlarging ova, and forced back into the bottom 
of the sac. 
In the interior of the acrocyst, the ova pass through certain stages of their 
development, and ultimately escape as free ciliated embryos by the rupture 
of its walls. 
In the cases just described, the acrocyst is destitute of any further covering, 
and has its walls with their gelatinous investment freely exposed to the 
surrounding water. In Sertularia rosacea and S. tamarisca, however, an addi- 
tional covering is provided for the acrocyst, and there is thus formed a curious 
and complicated receptacle, in which the ova, as in a sort of marsupium, pass 
through certain early stages of their development, previously to being dis- 
charged into the surrounding water. 
The nature and morphology of this receptacle in Sertularia rosacea (fig. 10) 
will be best understood by tracing its development. The young female gonan- 
gium is a conical body, with eight slightly projecting longitudinal ridges, and 
with the broad end of the cone constituting the distal end or summit of the 
gonangium. A blastostyle occupies its axis, having upon its sides, one over the 
other, the young budding gonophores, and expanding at its summit into a 
broad thick dise, which closes, as with a plug, the free end of the gonangium. 
Upon the outer side of this dise a thin chitinous investment is excreted, be- 
coming continuous at the edge of the disc with the chitinous walls of the 
gonangium, while in the centre of the dise the chitinous investment is defi- 
cient, leaving there a large circular aperture where the summit of the 
blastostyle is naked. 
The edge of the dise soon becomes produced into eight thick symmetrically 
radiating lobes, which gradually elongate themselves, carrying with them a 
continuation of the chitinous excretion, which forms a-wide tube around each ; 
and now bending upwards, in the form of eight arms with enlarged ex- 
tremities, they remind one of the disposition of the petals in a flower, and 
present altogether an appearance of great elegance. These eight radiating 
arms are composed of ectoderm and endoderm, and have their axis occupied 
* ”"Axpov, the summit, and céazts, a vesicle. Proc. Roy. Soc. Edin. 1858. 
