Chap. II. AURELIA FLAVIDULA. 23 



outer wall, being simple, irregularly polyhedral bodies without any obvious arrange- 

 ment among themselves. Normally the tentacles are developed by twos or a 

 multiple of two ; usually two begin first, and two more immediately follow, so 

 that in the outset the body has a quadrangular shape. In abnomral specimens 

 the embryo continues to grow for some time with only two papilliform tentacles 

 (PI. X. Ficj. 15 <?), although the base (c^) develops regularly, and the proboscis (c) 

 becomes quite prominent ; or, in others, the two tentacles increase considerably in 

 length {Fkj^. 16 and 16"), and the proboscis (e) grows very large. At other times, 

 the first two tentacles being far advanced, only a single additional one {^Figs. 17 e 

 and 18) develops on one side, and grows long before another appears on the oppo- 

 site side of the body. 



After four tentacles have developed, a considerable period elapses before any 

 others appear, and in the mean while all the different parts of the body progress 

 very far in growth ; the tentacles become very long and slender {Figs. 19, 20, 21, 

 and 22), and may be so extended as to more than equal the whole length of the 

 body. When stretched out thej'- are remarkably transparent, and allow a very clear 

 view of their interior structure (PL X". Fig. 2). The cells of the outer wall are 

 so merged into each other that their parietes are with difficulty made out, and 

 hence the transparent, film-like appearance of this wall. The inner wall of the 

 tentacles develops to such a degree that the component cells {Fig. 2 V") are as 

 fully characteristic in their appearance and conformation as in any of the later 

 stages of growth. The outer Avail of the body (PI. X. Fig. 19 «), now composed 

 of a single layer of cells (PI. X\ Fig. 6 a a^), is very thin and transparent. The 

 inner wall {Fig. 6 b 1?) is also composed of a single layer of very large cells, but 

 in this case they are totally different from those of the outer wall, having an 

 irregular, prismatic form, with the longer diameter transverse to the thickness of 

 the wall. This wall is five times thicker than the outer wall. The most striking 

 change observable since the last phase is in the mouth, which has assumed a 

 quadrilateral shape (PI. X. Fig. 22 c), as if it were four-lipped, each lip corre- 

 sponding to a tentacle. The extensibility of the lips may be seen as represented 

 in a figure {Fig. 21 c) showing the manner in which the young hydroid catches 

 its prey. This is the earliest period at which we have observed the embryo taking 

 food. The lasso-cells are in full activity, and their extruded threads give the 

 tentacles a bristling appearance (PI. X^ Fig. 2). In a state of complete expansion 

 the whole body is quite transparent, and has a uniform grayish color tinged Avith 

 orange, by the reflection from the pigment cells Avhich are scattered over the surface 

 of the digestive cavity. In a contracted state the croAvding of the pigment cells 

 renders the interior of the body quite opaque (PI. X. Fig. 19). The general con- 

 tour of the body, Avhen in full activity, is slender top-shaped {Figs. 20 and 21), 



