iv PHYLUM CCELENTERATA 101 



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 closes, the embryo being converted into a closed two- 

 layered sac or planula (Fig. 116, A), indistinguishable from that of 

 a Hydrozoon, although formed by a totally different process. 



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-ill or invagination of the surface so as to produce a 

 depression lined 'Ivith 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 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 tcvniolcs (D, tn.). 

 The mouth (E, intli.) 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 andF, 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 Leptolina3, but distinguished by a pronounced differentia- 

 tion of structure, indicated by the sixteen tentacles developed in 

 regular order, the stomodaeum, and .the four gastric ridges with 

 their septal funnels. The Scyphozoon-polype is called a Scyphula 

 or Scyphistoma. 



The Scyphula may grow to a height of half an inch, and some- 

 times multiplies by budding. After a time it undergoes a process 

 of transverse fission (G), becoming divided by a series of constric- 

 tions which deepen until the polype assumes the appearance of a 

 pile of saucers, each with its edge produced into eight bifid lobes, 

 four per- and four inter-radial. Soon the process of constriction 

 is completed, the saucer-like bodies separate from one another, 

 and each, turning upside down, begins to swim about as a small 

 jelly-fish called an Ephyrula (H, I). The umbrella of the ephyrula 

 VOL i M 



