Organs and Classification of A nimals. 9 



leave traces behind when their functions have ceased, 

 like the cord-like obliterated embryonic blood-vessels 

 in mammalia. In some animals, during their develop- 

 ment, organs spring into being for a shorter or longer 

 period, but never perform any function and either vanish 



FIG. 4. 



Larva of common Acorn-shell (Balanus porcatus), showing antennae (), 

 limbs (b\ and eye (d). 



or remain permanently in an undeveloped condition ; 

 such organs are called rudimental, and they are always 

 such as in some kindred form discharge an important 

 duty. Thus most cuttle-fishes have a groove in their 

 body during their embryonic life, which closes in and 

 forms a cavity wherein the internal shell is secreted; but 



