iv PHYLUM CCELENTERATA 173 



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 (1) derived from the endoderm, and on the outer 

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

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

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

 with each are two depressions, one on the ex-umbrella (plf. 1), the 

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

 depressions are lined with sensory epithelium and are called 

 oljactory 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. 129) differing in this respect, as well as in its mode of 

 formation, from the corresponding stage of a Hydrozoan. 



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, the process taking place 

 by a sinking-in or invagination of the surface so as to produce a 

 depression lined with ectoderm (B, s), the bottom of which 

 becomes perforated so as to communicate with the enteric cavity 

 (C, st) : the depression is the stomodceum, a structure of which 

 there is no trace in the Hydrozoa. On two opposite sides of the 

 mouth hollow processes 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 adradial tentacles 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 or tcenioles (D tn.). 

 The mouth (E, mth.') assumes a square outline, and its edges become 

 raised so as to form a short manubrium (mnb.} ; and, finally, the 

 ectoderm of the distal surface i.e. the region lying between the 

 mouth and the circlet of tentacles becomes invaginated in each 

 inter-radius so as to produce four narrow funnel-like depressions- 

 the septal funnels or infundibula (E and F, s. /.) sunk in the four 

 gastric ridges. 



The outcome of all these changes is the metamorphosis of the 

 planula into a polype (E), not unlike a Hydra or the hydrula-stage 

 of the Leptolinse, but distinguished by a pronounced differentia- 

 tion of structure, indicated by the sixteen tentacles developed in 



