AND DEVELOPMENT OF MYRIOTHELA. 
565 
They commence just below the hypostome, and extend for some distance backwards on 
the body. Springing from between the short permanent tentacles, and from a consi- 
derable portion of the body which lies at their proximal side, are the long arms (d, d, d) 
which made their appearance at an early period of embryonic development, and which 
are destined to disappear entirely before the arrival of the animal at maturity. They 
are about twenty in number, capable of great extension, and when stretched out to 
their utmost (fig. 6) are in the form of long straight filaments slightly tapering towards 
their distal extremities, where they terminate in a well-defined spherical capitulum. 
In complete retraction they are short, somewhat ovoid bodies marked by strong 
transverse rugae. 
In accordance with the terminology I have already adopted in describing the early 
stage of Tubularia *, I shall designate the free locomotive embryo of Myriotliela by the 
name of actinula. It moves about by the aid of its long arms, whose terminal capitula 
are capable of being used as suckers of attachment; while the proximal end of the body, 
like that of a hydra, also admits of being temporarily attached by means of its little 
suctorial disk. 
After the actinula has enjoyed for some days its free locomotive existence it begins 
to fix itself (fig. 7). This fixation is effected by means of the proximal sucker-like 
extremity ( b ). After it has thus become stationary it continues to manifest great contrac- 
tility, becoming sometimes much extended, and at other times contracted into a nearly 
spherical mass. The long arms now undergo a rapid degradation ( d , d, d) ; they lose 
their terminal capitula, become much shortened, and ultimately entirely disappear (fig. 8). 
In the mean time the short papilliform tentacles become more numerous, extending 
further backwards on the body. The proximal extremity of the animal becomes bent 
at right angles to the rest of the body so as to form a sort of horizontal stolon-like foot, 
from which small fleshy processes with sucker-like extremities, and having a considerable 
resemblance to the claspers, are emitted. The function of these processes, however, is 
very different from that of the claspers ; they serve to attach the animal permanently 
to some solid support, to which they fix themselves by their extremities. Along with 
the stolon-like foot they become clothed in a chitinous perisarc, and the actinula has 
thus acquired all the essential characters of the adult trophosome. 
The gonosome has not, however, as yet begun to develop itself ; but it soon makes its 
appearance by the budding of the blastostyles and claspers from the hydranth at the 
proximal side of the tentacles. From the blastostyles the gonophores are subsequently 
developed in the manner already described, and the animal thus attains its complete 
maturity (Plate 55). 
In the histological structure of the actinula there are several points which deserve 
special consideration. In the very young animal, at the time when the arms are about 
to become changed from internal to external appendages, the endoderm and ectoderm 
can be everywhere followed without difficulty. The endoderm is composed of clear 
* Gymnoblastic Hydroids, p. 90. 
4 F 
JIDCCCLXXV. 
