90 ANIMAL METAMORPHOSIS. 



body-cavity of their mother, and retained there until they have 

 developed their tentacles. The same process of development 

 takes place in the Corals. 



The Echinodermata, which include the sea-urchins, star-fishes, 

 crinoids, and sea-cucumbers, constitute one of the most interesting 

 divisions of the animal kingdom, and undergo some of the most 

 extraordinary changes in the passage from the ovum to the adult 

 animal, these changes forming one of the most remarkable sets of 

 objects to the microscopic enquirer. Their earl}' history was for a 

 long time unknown, but the problem was solved with great skill, 

 industry, and success by Johann Miiller. The striking character- 

 istic of their development is the formation of an intermediate or 

 " nurse " form — often wrongly called a larva — which is totally 

 unlike the parent, and does not attain the adult form by a process 

 of metamorphosis, but exists solely to give origin to the young 

 Echinoderm by a kind of internal gemmation (see PI. XI., Figs. 

 I, 2, 3). 



Take, first, the Echini, or sea urchins. Here, as with all 

 the Echinodermata, the first condition developed from the egg is 

 a ciliated, free-swimming Planula, which, by the formation of a 

 mouth and intestinal cavity, becomes a Gastrula. Presently, from 

 the gastrula is produced the intermediate form, which exists as a 

 sort of scaffold, on which the young Echinus is built up. So dif- 

 ferent is it from the parent that it was originally described as a 

 distinct organism, under. the name of Pluteus, so called from its 

 resemblance to a painter's easel (Fig. i). The body is composed 

 of colourless, transparent jelly; it is dome-shaped behind, ex- 

 panded, and slightly hollowed out in front, and prolonged 

 inferiorly into straight, slender legs, in which delicate rods of cal- 

 careous matter are perceptible, forming a kind of frame-work not 

 unlike the French clocks which we see on our mantel-pieces. It 

 has a distinct internal cavity, and propels itself by powerful cilia 

 grouped in two bunches on the sides of the body, resembling 

 epaulettes, and also by a fringe encirchng the dome, and continued 

 on all the columns up one side and down the other. After a time, 

 at the upper part of the body, a number of fine plates of lime 

 begin to form in the shape of a tiny round box. This gradually 

 extends over the gelatinous mass, and developes tubercles which 



