564 
PROFESSOR ALLMAN ON THE STRUCTURE 
derm, and which plainly corresponds to the planula of other hydroids. It is, however, 
entirely destitute of cilia, and is still confined within its external structureless capsule ( d ), 
which has now acquired considerable thickness. 
We next find that the planula presents numerous minute pits distributed without 
any definite arrangement over its surface (fig. 2, b, b). These are points where the 
walls of the planula have begun to invaginate themselves ; and if at this time a section 
be made of the planula (fig. 3), its cavity will be found to be occupied by numerous 
hollow conical projections (5, b ), which radiate into it on all sides from the inner surface 
of its walls. These projections are simple involutions of the walls, and are therefore 
composed, like the walls themselves, of an ectoderm and an endoderm, but in an inverted 
order. 
If an uninjured planula in this stage be dissected out of its external structureless 
capsule, which now lies loosely over it, and be subjected to carefully moderated pressure, 
the internal projections will become suddenly evaginated, and will shoot out in all 
directions over the outer surface in the form of hollow cylindrical arms. 
The evagination w T hich has thus been effected by artificial pressure takes place natu- 
rally in the progress of development ; and in the next stage (fig. 4) we find that the arms 
which had been formed internally by a process of involution have become external, the 
embryo being still enclosed within its capsule. The ectoderm had already, by the 
multiplication of its cells and the development in it of the clavate tissue, increased 
considerably in thickness, and the hyaline lamella may now be seen on its inner 
boundary. 
Up to this period the embryo had retained its nearly spherical form ; but it now begins 
to elongate itself, and assumes an oval shape (fig. 5). From its surface there project on 
all sides the tubular arms, which, from their original position within the cavity of the 
body, had become external by evagination ; while at one extremity of the greater 
diameter the body has become truncated, and here numerous short papilliform processes 
(i a ) have become developed from its surface. 
The arms continue to elongate themselves, and soon present a well-defined terminal 
capitulum. The papilliform processes, too, increase in number, and extend further 
back on the body of the embryo, which has become still more elongated. It is 
probably at this stage that the mouth is formed in the truncated end.' The embryo is 
now ready to escape from its enclosing capsule, which has all along remained adherent 
to the extremity of the clasper, and which now becomes ruptured, and allows the 
little animal to enter on a free life in the surrounding water (Plate 55. fig. 2, d d). 
The free embryo of Myriothela (Plate 58. fig. 6) is very contractile, and when fully 
extended is of nearly cylindrical form, about a quarter of an inch in length, slightly 
attenuated at one end so as to form a short conical hypostome («), which carries the 
mouth on its summit, and more decidedly so at the opposite end, where it terminates 
in a little sucker-like disk (b). The papilliform processes ( c ) have now attained the 
form of the permanent tentacles, presenting a short stem with a terminal enlargement. 
