ECOLOGY 275 



The marsupial pouch of the kangaroo and its allies is a pocket-like fold 

 of the integument, folded forward or backward over the region of the 

 abdomen in which are located the mammary glands. Hartman has recently- 

 described a very striking pie'ce of behavior in connection with the birth of 

 young opossums. The young are born in an exceedingly immature state 

 and looking like tiny pink grubs. They crawl under their own power, by 

 means of a swimming-like motion, through the hairs of the mother's 

 abdomen, till they reach the pouch. This they enter unaided and each tiny 

 "larva" finds for itself a slender tubular teat, which it swallows and holds 

 in place by a specially adapted hold-fast mouth. The young remains 

 attached fixedly to this teat for several weeks, feeding almost constantly 

 on milk. After a long interval the teat is released, the mouth metamor- 

 phoses into the adult form and the young feeds only at intervals, as do the 

 young of other mammals. This complex of adaptive structures and instincts 

 is among the most remarkable in the annals of biology. 



Nest-making instincts in birds represent, on the behavior side, adapta- 

 tions of extraordinary perfection. Some nests are built with the greatest 

 care and precision, others represent a relatively crude and slovenly per- 

 formance. Some nests are made of tvvigs, fibres, and mud, others of mud 

 alone, still others are hollowed out in clay or sand banks, and some are 

 made in holes in the ground. In any case, the type of nest is highly specific 

 and due to a hereditary instinct; for birds receive no instruction in nest 

 building. 



A vast number of animals and plants have given up the active search 

 for food and have taken up the relatively easy habits of parasitism. In 

 adaptation to this life certain structures have developed and many of the 

 characters found in independent, free-roving creatures have disappeared 

 or become reduced to mere vestiges. Thus the more completely dependent 

 or parasitic an animal becomes, the more completely does it lose its organs 

 of locomotion and its sense organs such as eyes, auditory organs, tentacles, 

 etc. Some animals are free-living when young or in the larval condition 

 and only settle down to a parasitic life when near the end of the life cycle; 

 other animals are parasitic only when young or larval and become inde- 

 pendent in the adult condition; still others are parasitic throughout the 

 entire life-cycle and pass from host to host without any interval of in- 

 dependent life. 



The classic case of extreme parasitic degeneration is that of SaccuUna, 

 a crustacean. The young larva swims about and leads a free life for a time, 

 but soon attaches itself by means of its antennae to a hair pit of a crab. The 

 internal tissues of the larva then undergo degenerative processes and are 

 reduced to an almost fluid mass of embryonic cells, which flow through 

 the hair pore of the crab and into the latter's lymph spaces. The small mass 

 of cells then rounds up and is carried about with the circulation of the 

 crab's blood until it comes to a favorable place of lodgment. Here it flattens 



