202 



PRINCIPLES OF ANIMAL BIOLOCY 



the adult form. Or they may possess organs which they must lose before 

 they become adults. Youn^ animals, leading a separate existence but 

 lacking certain organs of the adult, or possessing organs not found in the 

 adult arc known as lance. The offspring of jcUyfishes emerge from the 



Fig. 150. — The black swamp- wallaby. This is a marsupial, or pouched mammal, 

 found in Australia. The young are born in a very immature stage and are carried in a 

 pouch (marsupium) on the ventral side of the mother. For some time the young is at- 

 tached to the nipple in the marsupium, the nipple ftirming a bulb in the back part of the 

 mouth. Even after it is weaned and has attained a considerable size, the young when 

 alarmed seeks the pouch for protection (Photo loaned by the New York Zoolouical Society.) 



ovary of the mother, where as stated above the eggs are fertilized, as a 

 simple ball of cells, almost at the beginning of development. They 

 receive no care whatever thereafter. The embryos of sponges escape at 

 a stage almost as early as the jellyfishes. The developing embrj^os of 

 starfishes, sea-urcliins and their allies (Fig. 158) and marine worms are 



