1881.] The Siphonophores. 189 
pearance of the germinative vesicle altogether, leaving the egg 
perfectly homogeneous, and covered with short vibratile hairs or 
cilia, by the motion of which it is driven through the water. In- 
termediate changes, too technical to speak of in this account, 
occur, but I have omitted to mention them. Now opens a long 
chapter of the developmental history, which includes stage after 
stage, each different from its predecessor, following one another 
in rapid succession, all looking, although sometimes indistinctly, 
to the formation of a new Agalma. Of these stages in growth 
there are three which are characteristic and so distinct, that I 
have deemed them worthy of special names. They are of great _ 
importance in a study of the systematic position of Aga/ma, and 
are as follows:: 
1. Primitive larva or primitive medusa (Lizzia stage). 
2. Athorybia stage. ; 
3. Young resembling closely the adult, but still retaining embry- 
onic structures, z.e., embryonic tentacles, covering scales, &c. 
Between the primitive larva, the Athorybia stage and the 
adult Agalma, there is very little likeness. The third stage, 
however, has in most particulars a very close resemblance to 
the adult as figured in my sketch, Fig. 1, and differs from it only 
in size and in certain minor details. It is indeed very difficult to 
decide when the adult form of Agalma is really reached, for it 
begins to lay eggs when in an immature condition, as far as adult 
characters are concerned. 
The first change in the egg, after the peculiar process called 
Segmentation which I have already described, is the formation of 
a stage in which the germinative dot and vesicle disappear. A 
knowledge of this fact may be of use to one 
studying the process of development, for 
unless these structures in the egg of Agal- 
ma do disappear, the egg will not pass into 
following stages. Haeckel erroneously 
States that the germinative dot and vesicle 
does not disappear in genera closely allied 
and perhaps identical with Agalma. 
The next stage is the planula with the 
whole surface covered with cilia, which is 
followed by one in which is formed at one 
pole an elevation composed of two layers, which also becomes 
very thickly pigmented. The most superficial of these layers !s 
Fic. 7.—Egg of Agalma 
with apical elevation. 
