R Organs and Classification of A nimals. 9 

 uscular system, the blood-vessels, and the skeleton, 

 ihe embryo thus reaches its perfect state by under- 

 going a series of changes, which follow each other in 

 a definite order, and the study of these is called Em- 

 bryology. Some of the parts which appear during these 



FIG. 4. 



Larva of common Acorn-shell 

 (Balanns porcatns), showing an- 

 tennae (a), limbs (b), eye (d). 



Adult form of Balanns porcatus, 

 showing the absence of eyes and 

 feelers. Compare with fig. 4. 



processes have only a transient existence, and may dis- 

 appear e.g. the eyes of the free-swimming larva of the 

 common acorn shell (fig. 4), which vanish in the adult 

 (fig. 5) ; or they may leave useless tiaces behind them 



