172 ZOOLOGY 



SECT. 



position of the marginal notches and sense-organs. About the 

 base of each of the latter are special groups of nerve-cells. A 

 slight ring-like thickening of the plexus extends round the margin 

 in the neighbourhood of the marginal canal. 



The sense organs (Fig. 129) are lodged in the marginal 

 notches in close relation with the nerve-patches : like the latter, 

 therefore, four of them are per-radial and four inter-radial. Each 

 consists of a peculiar form of sense-club or tentaculocyst, containing 

 a prolongation of the circular canal, and thus representing a hollow 

 instead of a solid tentacle. At the extremity are calcareous con- 

 cretions or lithites (I) derived from the endoderm, and on the outer 

 side is an ectodermal pigment-spot or ocellus (oc). The 

 tentaculocysts are largely hidden by the marginal lappets (mg. Ip) 

 and by a hood-like process (h) connecting them ; and in connection 

 with each are two depressions, one on the ex-umbrella (olf. 7), the 

 other immediately internal to the sense-club (olf. 2) : these 

 depressions are fined with sensory epithelium and are called 

 olfactory pits. 



The development and life-history of Aurelia present several 

 striking and characteristic features. The impregnated egg-cell 

 or oosperm divides regularly and forms a morula, which, by accumu- 

 lation of fluid in its interior, becomes a blastula a closed sac with 

 walls formed of a single layer of cells. One end of this sac becomes 

 invaginated to form the gastrula. The blastopore or gastrula- 

 mouth does not completely close, the resulting two layered planula 

 (Fig. 130) differing in this respect, as well as in its mode of 

 formation, from the corresponding stage of a Hydrozoon. 



The planula swims about by means of the cilia with which its 

 ectodermal cells are provided, and, after a brief free existence, 

 settles down, loses its cilia, and becomes attached by one pole. 

 At the opposite pole a mouth is formed ; if, as represented in Figs. 

 B and (7, the process takes place by a sinking-in or invagination of 

 the surface so as to produce a depression lined with ectoderm (B, st), 

 the bottom of which becomes perforated so as to communicate with 

 the enteric cavity (C, st), the depression is the stomodceum, a struc- 

 ture of which there is no trace in the Hydrozoa : there is some doubt, 

 however, of the occurrence of any such ectodermal involution in the 

 case of Aurelia. On two opposite sides of the mouth hollow pro- 

 cesses grow out, forming the first two tentacles : soon two others 

 appear at right angles to these, the organism thus being provided 

 with four per-radial tentacles. Subsequently four inter-radial and 

 eight ad-radial tentacle's appear. At the same time the attached 

 or proximal end is narrowed into a stalk-like organ of attachment 

 (E), and the endoderm of the enteric cavity is produced into four 

 longitudinal ridges, inter-radial in position, and distinguished as 

 the gastric ridges oitcenioles (D, tn.). The mouth (E, mth.) assumes 

 a square outline, and its edges become raised so as to form a short 



