ANIMAL EVOLUTION 



53 1 



divide into two individuals which remain 

 attached in such a way that further 

 development results in the development of 

 a geometrically arranged assemblage. 



BILATERAL SYMMETRY 



There is still another alternative, which 

 is also foreshadowed in the single celled 

 animals or protozoans. These last, as 

 we know them now, are mostly asym- 

 metrical with a more or less definite 

 anterior end. In the same fashion a 

 multicellular animal may start to develop 

 in radially symmetrical fashion, but 

 during the course of its development lose 

 the radial symmetry and become bi- 

 laterally symmetrical with an anterior or 

 head end, toward which naturally the 

 mouth and chief sense organs converge 

 and at which the controlling nerve centers 

 become assembled. 



With a bilaterally symmetrical, more or 

 less elongated form and a head end at 

 which are situated the controlling nervous 

 centers and sense organs and the mouth, 

 and endowed with locomotion, an animal 

 becomes independent of its immediate 

 surroundings. It is able to search for 

 food and thus to get sufficient nutriment 

 to enable it to grow to almost any size. 



What evidence is there that the bi- 

 laterally symmetrical animals have any 

 connection with radially symmetrical 

 types? The gastrula — the two layered 

 cup — or its equivalent is found in all 

 animals except those composed of a single 

 cell, and those composed of an unorganized 

 mass of cells — the sponges. The latter 

 are singularly diversified in their early 

 stages, but these never include a gastrula 

 comparable to that in the other multi- 

 cellular animals. 



The gastrula is the pre-adult stage in 

 the hydra, the sea-anemones, the corals, 

 the sea-pens, and their allies. In all other 

 animal types it is the stage at which 



divergence takes place in all directions. 

 In the flatworms and in the roundworms 

 the adults show a bilateral symmetry 

 with much of the original radial sym- 

 metry still remaining, but in all the 

 other forms of animal life all traces of 

 radial symmetry are lost. 



All animals arise from a single cell; 

 therefore we say that the single cell is the 

 fundamental feature of the structure of all 

 animals. But if this is true, then it is 

 equally true that the gastrula, or the 

 radially symmetrical element, is the fun- 

 damental feature of the structure of all 

 the bilaterally symmetrical animals. 



Especially to be remarked in the strictly 

 bilateral animals is the constant recurrence 

 of features characteristic of the radially 

 symmetrical forms, such as the formation 

 of colonies, as in the polyzoans, and repro- 

 duction by budding or division, as in 

 some starfishes and ophiurans, and in many 

 other types. 



The constant recurrence of these features 

 may mean either of two things. There 

 may be a natural tendency in every animal 

 group to adopt independently a colonial 

 habit and a propensity to reduplication 

 by simple division of the body after the 

 manner of the radially symmetrical types, 

 or the constant reappearance of these 

 features may be due to an inherent princi- 

 ple common to all animals and inherited 

 from a common origin in which these 

 features dominated. 



Generally speaking the tendency for 

 animal types to form colonies or to repro- 

 duce by budding is inversely proportionate 

 to their activity; that is, the more active 

 the animals, the less tendency there is to 

 produce colonies, or to reproduce by buds 

 or by division. 



Among the crustaceans there is only a 

 single type (Thompsonia) that may prop- 

 erly be considered as colonial, while 

 relatively few reproduce by budding in the 



