Chap. II. STRUCTURE OF THE ADULT. 51 



{c e). Its cells are very tliin walled, irregularly polyhedral (/), and have perfectly 

 homogeneous, hyaline contents ; they are capable of great elongation («^) or of 

 contraction («^), and are largest in the region of the lasso-cells (a^), which are 

 imbedded among them, in large numbers, and in all stages of development. The 

 tip of the tentacle especially (c) is crowded with lasso-cells; in fact they seem to 

 be the only constituents of the outer wall, so closely are they packed. As they 

 are arranged at pretty regular intervals, in groups, all around the tentacle, they 

 give it a knotted appearance, which in the adult becomes a very marked feature 

 (PI. VII. Fiffs. 2, 3, and 4). The inner wall {b P- V^) has a more uniform thick- 

 ness, which is dependent alone on the amount of expansion or contraction of the 

 tentacle. Like those of the outer wall, the cells of this are capable of great 

 elongation [b^) or of extreme contraction, and have perfectly homogeneous contents. 

 The surface of the disk is studded with collections of lasso-cells {Figs. 9 o and 7 a h), 

 which as yet only number about a dozen in each group. The epithelial cells {Fir/. 

 7 c) have very thick walls in a horizontal direction, and numerous young cells are 

 developing between them. 



In this condition, the young Aurelia resembles the adult so closely in its gen- 

 eral appearance, that it is hardly worth while to trace further, step by step, the 

 successive enlargement of the whole body up to its mature condition, as this 

 would lead to frequent unnecessary repetitions, inasmuch as from this time forwards, 

 some parts undergo hardly any changes, while others only increase in number, and 

 only a few new features are introduced. It may, therefore, suffice now to describe 

 the adult and to allude incidentally to the final transformation of all its parts. 



SECTION III. 



STRUCTURE OF THE ADULT AURELIA ELAVIDULA. 



The body of all Acalephs consisting of a repetition of identical parts, symmetri- 

 cally arranged around a vertical axis, and yet variously combined with one another, 

 it is indispensable to consider this arrangement first, in order to form a correct 

 idea of their structure. In Aurelia, in their adult state, the most conspicuous parts 

 are the gelatinous body or disk, the indentations along its margin, the crescent-shaped 

 organs around the centre, and the prominent appendages on the under side ; and, 

 though the number of these parts varies occasionally, there are usually eight indentar 

 tions along the margin, four crescent>shaped bodies near the centre, and four large 

 appendages below. The variations in number arise from the interpolation of similar 



