53 z 



THE QUARTERLY REVIEW OF BIOLOGY 



early stages; all of these are sedentary- 

 forms. Among the insects a few types 

 exhibit the phenomenon known as poly- 

 embryony — the formation of two or more 

 larva; from a single egg — but quite a 

 number, as ants, bees, wasps, and termites, 

 the last named radically different from the 

 others and related to the cockroaches, 

 form colonies which, although the in- 

 cluded individuals are all separate, show 

 unmistakable resemblances to ccelenterate 

 and other colonies. In the vertebrates, 

 all of which are of large size and therefore 

 require a large amount of food, these 

 phenomena have almost completely dis- 

 appeared, as would be expected. The 

 remarkable powers of reparation possessed 

 by certain amphibians are possibly a rem- 

 nant of reproduction by budding, while 

 the ceratioid fishes with the diminutive 

 males parasitic on the females might be 

 considered as colonial in habit, though 

 beginning life as two or more wholly 

 distinct individuals. 



Animal colonies may form a single unit 

 which may acquire locomotor powers 

 and act exactly as if it were a single 

 animal. Examples of this are the colonial 

 jelly-fishes known as siphonophores, of 

 which the Portuguese man-of-war (Phy- 

 salia) is an illustration, swimming 

 colonies of tunicates (Pyrosoma), and 

 crawling colonies of polyzoans (Cristatella). 



Among animal types true radial sym- 

 metry is relatively rare. Most of the 

 protozoans are more or less asymmetrical, 

 and most of the coelenterates or ' 'radiated 

 animals" also are to a greater or lesser 

 degree asymmetrical. 



A tendency toward radial symmetry, 

 more or less marked, occurs sporadically 

 throughout the groups of animals which 

 are primarily bilaterally symmetrical. 

 Thus the adult sea-urchins, sea-lilies and 

 feather-stars, starfishes, brittle-stars and 





holothurians, making up the group known 

 as the echinoderms, are usually more or 

 less perfectly radially symmetrical, al** 

 though when young all echinoderms are 

 bilaterally symmetrical. 



Among the single celled animals or 

 protozoans some are attached and more or 

 less radially symmetrical, like Stentor oft 

 Vortkdla; others are attached and form 

 colonies on the summit of a stalk, like 

 Codosiga or Epistylis; others, as Amceba, are 

 free living and capable of locomotion, but 

 are sluggish and creep equally well in 

 any direction; some float freely suspended 

 in the water; while others are elongated 

 and more or less bilaterally symmetrical 

 and swim with great rapidity. Some 

 are naked, while others form beautiful 

 regular and complicated shells of lime or 

 other substances, or rough agglutinated 

 tubes of sand grains, or other forms of 

 covering. 



All of these varied characters are dupli- 

 cated among the radially symmetrical 

 animals, and again in those derived 

 through a gastrula stage, this duplication 

 being rendered possible because the great 

 difference in size prevents direct con*- 

 petition. 



We need not carry this recitation 

 further. It would appear that among the 

 animals taken as a whole every possible 

 response to the physical environment 

 occurs, and furthermore is repeated in 

 widely different types whenever because 

 of a difference in size or for other reasons 

 there can be no direct competition. 



We can scarcely doubt, therefore, that 

 in the first place animal life taken asw 

 whole forms a unit which is very closely 

 knit, with the component types far mote 

 intimately joined each to the other than 

 is commonly supposed, and in the second 

 place that the majority of the obvious 

 differences between the component types 



