THE TUBE-WITHIN-A-TUBE BODY PLAN 



175 



with or without ramifications. All the food 

 makes its way into such a digestive tract 

 through the mouth, and all undigested food 

 comes out the same way. This is a very 

 awkward method of handling such an im- 

 portant function. A distinct improvement 

 would be a tube running throughout the 

 body with the mouth at one end, and a cor- 

 responding opening at the opposite end to 

 allow undigested food to pass out of the 

 body. In this way food could be constantly 

 taken in at one end, and progressively di- 

 gested as it passes backwards, a kind of as- 

 sembly line method. This would mean the 

 development of a "tube-within-a-tube" body 

 plan. 



Another great need was a method of dis- 

 tributing the digested food to all the cells 

 of the body. So far this had been done by 

 simple absorption and diffusion. To be sure, 

 certain provisions had been made to facil- 

 itate this process, but at best, such things as 

 the diverticula in the gut of planaria, could 

 suffice for only a very small animal. Such 

 primitive devices for distribution could not 

 supply all the cells fast enough to make it 

 possible for the animal to reach any great 

 size or to move with any speed. A system 

 was required which would carry an ample 

 supply of not only food, but also oxygen to 

 burn it, to every cell. This could be done 

 only with some sort of conveyor belt system. 

 Since digested foods are soluble in water, 

 the system must be made up of a circulating 

 fluid, a series of tubes to confine and lead it 

 to every cell, and some means of keeping 

 it flowing continuously. Only with the de- 

 velopment of such a mechanism could ani- 

 mals climb any higher in this scale. 



The first of these important steps was 

 taken by the animals found in the phylum 

 Nemathelminthes, the roundworms, and the 

 second step among an obscure group of ani- 

 mals, the Nemertinea. These two groups 

 will be studied from this point of view, and 

 in this order, although in most other re- 

 spects the nemertines are more primitive 

 than the nematodes. 



PHYLUM NEMATHELMINTHES 



In numbers of animals the nematodes, 

 which is the name applied to most members 

 of the phylum Nemathelminthes, perhaps 

 exceed all others, with the possible excep- 

 tion of the arthropods and Protozoa. They 

 were once thought to be primarily parasitic. 

 These members first came to the attention 

 of biologists because they were responsible 

 for some of the more serious diseases both 

 in plants and animals. However, it is now 

 known that there are equally as many, if not 

 more, that are free-living. A spadeful of al- 

 luvial soil contains literally millions of them. 

 A drop of water, taken from nearly any stag- 

 nant pond or the sea, would reveal many of 

 them. Their characteristic whipping move- 

 ment identifies them to even the casual 

 observer. Most of the nematodes are very 

 small, almost microscopic, although there 

 are a few — the "horsehair worms," for ex- 

 ample — that may reach a length of 1 yard. 

 Some of the ascarid parasites of horses may 

 reach a length of 10 to 12 inches. 



The most distinct improvement in this 

 group over the preceding is the complete 

 digestive tract, mouth to anus (Fig. 10-1). 

 This is a slender tube, without pockets, run- 

 ning throughout the body length. Digested 

 food is absorbed directly through the gut 

 wall and diffuses into a fluid which sur- 

 rounds the digestive tract, thence to the 

 body cells. Here again, the animal depends 

 on diffusion to take care of the important 

 matter of getting food and oxygen to the 

 cells, and wastes away from them. This fact, 

 among other things, is probably responsible 

 for the small size of these animals. 



A pair of tubes run internally along each 

 side of the body, forming excretory canals, 

 but they lack any cells comparable to the 

 flame ceUs found in planaria. The two tubes 

 unite into a single one, which opens ven- 

 trally to the outside through a minute ex- 

 cretory pore. Another feature of the nema- 

 todes is a complex nervous system which 

 consists of several nerves extending the 



