natural objects there is some marvel, and if anyone 

 despises the contemplation of lower animals, he must 

 despise himself." From Aristotle's time to our own 

 there have always been some minds that feel chal- 

 lenged by whatever is unknown, especially if it causes 

 vast human suffering. The unraveling of the com- 

 plexities of flatworm structure and habit is fortu- 

 nately a very active field of modern research. 



Soft-bodied animals that are several to many times 

 as long as they are wide are inevitably tagged as 

 worms, and this name has been applied to soft, elon- 

 gated members of practically every large grouping of 

 animals. Of all the kinds of worm-shaped creatures, 

 the members of the phylum Platyhelminthes ("flat 

 worms" ) are on the whole the most flattened and the 

 most primitive. The digestive cavity, when present at 

 all, has only one opening, as in the coelenterates. In 

 place of the jelly that provides much of the coelen- 

 terate bulk, however, flatworms have a solidly cellu- 

 lar middle layer, which includes several sets of mus- 

 cles and a variety of organs, especially of reproduc- 

 tive organs, a specialty of these animals. 



With few exceptions, the flatworms are hermaph- 

 roditic — that is, each individual produces both eggs 

 and sperms. This does not mean that self-fertilization 

 is the rule. On the contrary, most flatworms are en- 

 dowed with an amazingly complex set of organs for 

 exchanging sperms with their neighbors or chance 

 acquaintances and for storing the sperms toward the 

 time when their eggs are to be fertilized. The ferti- 

 lized eggs, enclosed in delicate capsules or in hard- 

 ened shells, are shed to the exterior, and by means 

 of adhesive secretions may be strung together in egg 

 ribbons or masses or attached singly to stones or other 

 objects. Some of the fresh-water flatworms are espe- 

 cially noted for their ability to multiply asexually by 

 fragmentation or by crosswise rupture of the body. 

 This has led to detailed studies of their ability to re- 

 generate when experimentally cut into small pieces. 



Beginning with the flatworms, all the groups of 

 animals are two-sided or bilaterally symmetrical. Or 

 they have some secondary modification of that kind 

 of symmetry. Bilateral animals have a front end that 

 goes first when the animal moves, and a rear or tail 

 end that follows along. They also have differing up- 

 per and lower surfaces, and right and left sides that 

 mirror each other. Organs that occur singly are usu- 

 ally in the mid-line, and paired organs occur on each 

 side of the mid-line as in ourselves. This means that 

 the flatworms are the first animals with a head. The 

 major sense organs are concentrated on the head or 

 front end, and most of the animal's wits are gathered 

 into a brain, a concentration of nerve cells in the 

 head. Speedier, more coordinated behavior is the re- 

 sult, with more rapid responses to prey or enemies 

 than in the radial coelenterates. 



The free-living flatworms have a highly developed 



talent for clinging to surfaces, and some fresh-water 

 planarians even have well-developed muscular suck- 

 ers for holding on. So it is not surprising that flat- 

 worms eventually took up parasitic habits and pro- 

 duced the formidable array of suckers and hooks by 

 which the various flukes and tapeworms maintain 

 their tenacious hold on the hosts that nurture them. 



The Free-livino; 



Flat 



worms 



{Class Turbellaria) 



The free-living flatworms are at least partially 

 clothed with cilia that propel the smaller forms and 

 the young stages of larger members. In water these 

 cilia create the turbulence that suggested the name 

 of the group. The larger turbeUarians, whether 

 aquatic or terrestrial, glide along primarily by mus- 

 cular waves, though these may be invisible to the 

 naked eye. To ease their way, land turbeUarians 

 must lay down a thick carpet of secreted mucus, over 

 which they glide smoothly or sometimes hurry by a 

 more energetic series of muscular contractions. Even 

 the aquatic forms use a mucous bed, especially over 

 rough surfaces. 



Shapes vary from elongated cylindrical worms 

 to extremely thin and flattened leaflike marine forms 

 that are almost circular. Though a few have tail lobes, 

 or little sensory lobes or tentacles on the head, these 

 are for the most part streamlined little animals with 

 no projections. 



A very few turbeUarians are parasitic, and some 

 are internal or external commensals that share the 

 food of their hosts while doing no serious harm. Most, 

 however, are carnivorous, eating tiny animals of suit- 

 able size or working away, bit by bit, at large pieces 

 of dead flesh or at living sessile animals, such as oys- 

 ters or barnacles, that cannot flee. Land planarians 

 can subdue insect larvae, snails, or even earthworms. 



TurbeUarians are divided into five orders based 

 primarily on differences in the form of the digestive 

 cavity; this internal distinction can often be readily 

 seen through the transparent body wall. 



THE ACOELS 



The name "acoel" means "without a cavity," and 

 these minute and delicate worms have no digestive 

 cavity. The mouth, usually in the center of the under 

 surface, directs the food into the inner mass of cells, 

 where it is digested. Acoels are exclusively marine, 

 and most of them are elongate or broadly oval and 

 measure from K-, to 's of an inch in length. They 

 live so inconspicuously under stones, among algae, 

 on muddy bottoms, and sometimes on sandy shores, 

 that they are seldom seen by anyone not actively 



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