APPENDIX. 397 
tion in Nature and Human Life.” 9? (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 hydromedusz 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 acolumn 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 medusz, the disc-jellies (Discomeduse, vol. li. 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 primeval polyps, 
differing very little from the common fresh-water polyp (Hydra). 
The alternation of generation in these Discomedusz 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. 11. p. 142), has two representatives on Plate VII. 
To the left, in the centre, between the A%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 
