SYSTEMATIC DEVELOPMENT. 



57 



Medusae (comp. 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 ani- 

 mal resembles the permanently sessile genera, which, by 

 all systematic rules, and by their geological position, 

 occupy a lower rank in the series of echinoderms. The 

 crabs, or anourous Crustacea, are raised by sundry char- 

 acteristics above their long-tailed congeners, among 

 which is the fresh-water crayfish. In the course of de- 

 velopment they pass through the long-tailed stage, as is 

 shown in the larva (Fig. 8). It is by the abortion of the 

 tail, which is employed by the long-tailed species as a 

 natatory organ, that they become more fitted for run- 

 ning, 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 Verte- 

 brata, 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 mam- 

 mals and to man, they have, nevertheless, carried it a 

 stage further than the reptiles, for the scale is the em- 

 bryonic rudiment of the feather. Likewise, the tarso- 

 metatarsal 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 rapidly 



