SYSTEMATIC DEVELOPMENT. 57 



Medusas (co np. Fi^. 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, w^hich 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 Vcrtebrata, 

 leads through the reptiles to the birds. Now, if, in the 

 physiological reflections which Von Baer put into their 

 beaks, the birds, as v/ill 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 



