88 ANIMAL METAMORPHOSIS. 



which are indistinguishable from certain of the flagellated monads. 

 These soon lose the cilia, and pass through an intermediate form, 

 in which they closely resemble an ordinary amoeba. Ultimately, 

 these amoeboid particles run together, forming a compound zygo- 

 spore, from which fresh spores are again developed. Other forms, 

 though passing through an equally distinct series of metamorpho- 

 ses, retain their animal characteristics throughout life. 



Among all animals of higher organisation than the Protozoa, 

 we find that the processes of gemmation and fission are almost 

 invariably accompanied by the production of ova. The first step 

 in development after the fertilisation of the ovum by the sperm-cell 

 is the division of the yelk into two. In like manner, these two 

 cells, by continued sub-division, separate into four, these into 

 eight, and so on. Each of the outside cells now puts forth a 

 minute cilium, or lash, so as to form a fringe round the whole 

 body, and the embryo, known as a " primula," is capable of free 

 locomotion, and so exceedingly like an infusorian that there is no 

 doubt many of those supposed organisms are nothing more than 

 the early condition of more advanced forms into which they ulti- 

 mately develop themselves. In the next stage the cilia vanish; a 

 central cavity is formed, which communicates with the exterior by 

 an aperture placed at one end. It is now called a gastrula. 

 The gastrula stage, according to Haeckel, is passed through during 

 the development of all tribes of animals, (the lowest group of the 

 Protozoa alone excepted) but in the higher orders it is only an 

 embryonic condition, before the individual is capable of a sepa- 

 rate existence ; while the Sponges, Hydrozoa, Coelenterata, and 

 Annulosa all spend the first independent stage of their life as free- 

 swimming planulae and gastrulae, passing by various processes of 

 metamorphosis to their full development. Thus the cihated 

 embryos of Sponges and Hydrae, after freely swimming about for 

 some time, find a suitable locality, and fix themselves to some 

 solid object. In the young sponge, spicule begin to appear and to 

 be aggregated into bundles, and they are ultimately developed 

 into perfect Spongillae ; while the young Hydra developes a row of 

 tentacles round the mouth, and takes on the form of the adult 

 animal. 



In some of the Compound Hydrozoa the process is still more 



