APPENDIX. 387 



tion in Nature and Human Life." ^'^ (Compare vol. i. p. 270, and 

 vol. ii. p. 140). An example of this is given in Plate VII. in 

 the drawing of the beautiful Physophora (13). This swimming 

 stock or colony of hydromedus^ is kept floating on the surface 

 of the sea by a small swimming bladder filled with air, which in 

 the drawing is seen rising above the surface of the water. Below 

 it is a column of four pairs of swimming bells, which eject water, 

 and thereby set the whole colony in motion. At the lower end of 

 the column of swimming bells is a crown-shaped wreath of curved 

 spindle-shaped sensitive polyps, which also serve as a cover- 

 ing, under the protection of which the other individuals of the 

 stock (the eating, catching, and reproductive persons) are 

 hidden. The ontogenesis of the Siphonophora (and especially of 

 this Physophora), I first observed in Lanzerote, one of the 

 Canary Islands, in 1866, and described in my " History of the 

 Development of the Siphonophora," and added fourteen plates for 

 its explanation. (Utrecht, 1869). It is rich in interesting facts, 

 which can only be explained by the Theory of Descent. 



Another circumstance, which is also only explicable by the 

 Theory of Descent, is the remarkable change of generation in the 

 higher medusae, the disc- jellies (DiscomedusaB, vol. ii. p. 136), a 

 representative of which is given at the top of Plate VII., in the 

 centre (rather in the back ground), namely, a Pelagia (14). 

 From the bottom of the bell-shaped cup, which is strongly arched 

 and the rim of which is neatly indented, there hang four very 

 long and strong arms. The non-sexual polyps, from which these 

 disc-jellies are derived, are exceedingly simple primaeval polyps, 

 differing very little from the common fresh- water polyp (Hydra). 

 The alternation of generation in these Discomedusae has also been 

 described in my lecture on Differentiation,^'' and there illus- 

 trated by the Aurelia by way of example. 



Finally, the last class of Zoophytes, the group of comb-jellies 

 (Ctenophora, vol. ii. p. 142), has two representatives on Plate VII. 

 To the left, in the centre, between the ^quorea (9), the Phy- 

 sophora (13), and the Cunina (12), is a long and thin band 

 like a belt (15), winding like a snake; this is the large and 



