40 INTRODUCTION TO ZOOLOGY CHAP. 



becomes quadrangular, and four folds of tissue grow inwards 

 from the body-wall, forming the four " gastric ridges " which 

 alternate in position with the angles of the mouth. From 

 these gastric ridges arise the sexual organs and gastric fila- 

 ments described previously. 



Above each gastric ridge, the surface of the mouth-cone 

 becomes pushed in to form a narrow funnel-shaped pit. The 

 final result is the formation of a little polyp, not very unlike 

 a small Hydra; this polyp is the Scyphistoma (Fig. 19, A). 

 In the autumn each such Scyphistoma gives rise, by a curious 

 process of repeated transverse constriction, to a whole series of 

 little embryo jelly-fishes, each of which is known at first as 

 an Ephyra. First there appears in the hydroid body below 

 the tentacles a series of ring-like constrictions which deepen 

 until the polyp begins to look like a pile of very deep minute 

 saucers, the margin of each saucer becoming deeply eight- 

 lobed (see Fig. 19, B). This stage is known as the Strobila. 

 After a time the constriction reaches right across the polyp, 

 and the saucer-like segments, one after the other, gradually 

 separate from the polyp, turn upside down, and swim off as 

 tiny shallow jelly-fishes or Ephyrae. As the constriction has 

 proceeded a portion of the gastric cavity with the gastric 

 ridges has been nipped off in each Ephyra. This cavity is 

 closed below when the constriction is completed, but, on the 

 side which is uppermost before it is set free, it is drawn out 

 into a little mouth-tube, ending in an open mouth. This mouth- 

 tube can be seen through the side of the transparent Ephyra in 

 Fig. 1 9, C. The curve of the surface is at first very slight, but 

 rapidly increases ; the portions between the lobes grow out so 

 that the margin becomes circular, the marginal tentacles de- 

 velop, the gastric canals form, and the mouth-lobes protrude ; 

 thus the Ephyra gradually assumes the adult form shown in 

 Fig. 16. The medusae of many other jelly-fish develop 

 straight from the egg without this complicated process of 

 the transverse fission of a hydroid ; this is so in the common 

 Pelagia. 



PRACTICAL WORK ON THE JELLY-FISH (Aurelia) 



1. These animals should be studied alive in their natural 

 habitat, since after death their soft bodies lose their beauty of 

 form and colour, and change their shape very greatly. 



