390 APPENDIX. 
Plate IX. shows the fully developed and sexually mature 
animals of the second generation from the mouth side, which, in 
the natural position of Star-fishes (when creeping at the bottom 
of the sea), in sea-stars (A 6) and sea-urchins (C 6), is below, 
in sea-lilies (B 6) above, and in sea-cucumbers (D 6) in front. ° 
In the centre we perceive, in all the four Star-fishes, the star- 
shaped, five-pointed opening of the mouth. In sea-stars, from 
each arm there extend several rows of little sucking feet, from 
the centre of the under-side of each arm to the end. In sea- 
lilies (B 6), each arm is split and feather-like from its base up- 
wards. In sea-urchins (C 6) the five rows of sucking feet are 
divided by broader fields of spines. In sea-cucumbers, lastly 
(D 6), on the worm-like body it is sometimes only the five rows 
of little feet, sometimes only the feathery tentacles surrounding 
the mouth, from five to fifteen (in this case ten), that are exter- 
nally visible. 
(Piates X. anp XI. (Between pages 174 and 175, Vol. IT.) 
Historical Development of the Crab-fish (Crustacea).—The two 
plates illustrate the development of the different Crustacea from 
the nauplius, their common primeval form. On Plate XI. six 
Crustacea, from six different orders, are represented in a fully 
developed state, whereas on Plate X. the early nauplius stages are 
given. From the essential agreement between the latter we may, 
on the ground of the fundamental law of biogeny, with full 
assurance maintain the derivation of the different Crustacea 
from a single, common primary form, a long since extinct 
Nauplius, as was first shown by Fritz Muller in his excellent 
work “ Fiir Darwin.” 
Plate X. represents the early nauplius stages from the ventral 
side, so that the three pairs of legs, on the short, three-jointed 
trunk are distinctly visible. The first of these pairs of legs is 
simple and unsegmented, whereas the second and third pairs 
are forked. All three pairs are furnished with stiff bristles, 
which, through the paddling motion of the legs, serve as an 
apparatus for swimming. In the centre of the body, the per- 
