THE TUBE-WITHIN-A-TUBE BODY PLAN 



due to hemoglobin, which makes up the 

 major portion of the material found in these 

 cells. The affinity of this pigment for oxygen 

 makes it useful in an oxygen-carrying ca- 

 pacity. 



Another deficiency of the nemertine cir- 

 culatory system is the lack of a pumping 

 station. The only propulsive force for the 

 blood is furnished by the contractions of 

 the animal as it swims. This massaging; ef- 

 fect causes the blood to flow along in the 

 vessels. These deficiencies of the blood 

 system are veiy great, which is perhaps 

 one reason why these animals have never 

 achieved greater success. But imperfect as 

 it is, it is a great step forward over the more 

 primitive arrangements for distribution. 



PHYLUM ANNELIDA 



Not only are the features initiated in ear- 

 lier phyla further developed in the phylum 

 Annelida, but new features are also intro- 

 duced. Two principal ones are segmenta- 

 tion, or metamerism, and the formation of 

 a body cavity, the coelom. These two in- 

 novations have made it possible for animals 

 to grow larger bodies and to develop more 

 complicated neuro-muscular mechanisms, 

 thus permitting greater and more diversi- 

 fied activity. 



The most obvious difference between an 

 annelid and lower forms is its segments. 

 These are serially similar parts, conspicuous 

 both on the outside and the inside of the 

 body. Internal organs are repeated in every 

 segment, each part resembling all others 

 in most respects. There are modifications, 

 however, in certain body regions, as we 

 shall see. There is an intricate connection 

 between the segments, so that the animal is 

 a coordinated whole. Segmentation is re- 

 tained in higher forms such as the arthro- 

 pods and the chordates, and is therefore a 

 successful idea that has contributed toward 

 the greater complexity of animal bodies. 

 Just how segmentation came about is con- 

 jectural. It may have resulted simply from 



185 



individuals dividing asexually by transverse 

 fission, remaining attached, and eventually 

 becoming integrated into a coordinated 

 whole. Phenomena leading to support this 

 explanation can be found among some of 

 the flatworms (see p. 79). 



The members of the phylum Annelida are 

 mostly free-swimming marine worms. They 

 abound in the oceans and live near the 

 shore and hide in the sand in burrows dur- 

 ing their quiet periods. Some biologists 

 believe that the annelids are intermediate 

 between the lowest protozoan forms and 

 the highest vertebrate animals; in other 

 words, they represent the halfway mark up 

 the long path of evolution. Since they form 

 the basis for further development, it is nec- 

 essary to examine this group of animals 

 carefully, which we shall do by studying 

 two representatives, the sandworm (Nean- 

 thes), and the common earthworm (Liim- 

 briciis). Of these Neanthes is more typical 

 because it is aquatic and possesses more 

 of the characteristics of the phylum. The 

 earthworm, on the other hand, is terrestrial 

 and in many respects is quite unlike most 

 annelids. 



Neanthes 



Neanthes (formerly known as Nereis), 

 the common "sandworm" or "clamworm" 

 (Fig. 10-9), lives in shallow water on the 

 sandy shores of most oceans of the world, 

 where it is found in small burrows with its 

 head and tentacles protruding slightly. 

 When small animals venture too close, the 

 worm suddenly everts its heavily armed 

 proboscis, seizes its prey, and drags it into 

 the burrow to be devoured. When at rest 

 the worm actually "stands" in its burrow 

 and undergoes a constant undulating move- 

 ment, creating a current of water that flows 

 in and out for breathing. The worm leaves 

 its burrow during the breeding season but 

 only rarely does it leave otherwise. 



The segments of Neanthes are conspicu- 

 ous externally, especially because each one 

 possesses a pair of laterally placed paddles, 



