SYSTEMATIC DEVELOPMENT. 57 
Medusz (cio np. Fig. 3, p. 43), a polype-like condition is 
interposed. The crinoid (Comatula), very common in 
the Mediterranean, is in its mature condition freely 
movable. This definitive development is, however, pre- 
ceded by a sessile stage (Fig. 7), during which the 
body is attached to a stalk. During the larval period 
the animal resembles the permanently sessile genera, 
which, by all systematic rules, and by their geological 
position, occupy a lower rank in the series of echino- 
derms. The crabs, or anourous crustacea, are raised by 
sundry characteristics above their long-tailed congeners, 
among which is the fresh-water crayfish. In the course 
of development they pass through the long-tailed stage, 
as is shown in the larva (Fig. 8), It is by the abor- 
tion of the tail, which is employed by the long-tailed 
species as a natatory organ, that they become more 
fitted for running, and some of them for terrestrial life, 
as they are, in a measure, released from a burden. 
One of the systematic series included in the Vertebrata, 
leads through the reptiles to the birds. Now, if, in the 
physiological reflections which Von Baer put into their 
beaks, the birds, as will appear later, were mistaken in 
boasting of their feathery garb in contrast to mammals 
and to man, they have, nevertheless, carried it a stage 
further than the reptiles, for the scale is the embryonic 
rudiment of the feather. Likewise, the tarso-meta- 
tarsal joint of the embryonic bird, with which we are 
already conversant (p. 9), and which is distinguished 
from the ankle-joint of mammals and of man, by its 
lying not between the leg and the tarsus, but in the 
tarsus itself, remains, as a definitive condition in the 
reptile, in the embryonic condition which in the bird it 
