Evolution 



by the main nerve cord in the vertebrates, an important point 

 of difference. 



The Arthropods are an extraordinarily successful group. A 

 multitude of forms of Crustaceans populate the waters, and they 

 are excelled in numbers and variety only by the insects upon 

 land. While the individual size appears to be somewhat strictly 

 limited, probably by the nature of the respiratory and blood 

 systems, many types show exceedingly high development in 

 various directions — in intelligence, in social and parental instincts, 

 etc. The insects are of course to be regarded as the highest 

 Articulata, and have, like the highest vertebrates, the mammals 

 and birds, almost completely forsaken the water for the dry land 

 and the air. An interesting member of the Articulata, from 

 the standpoint of the Evolution theory, is the Peripatus, shown 

 with a ringed worm on Fig. 52 (3). It gains its interest for us 

 from the fact that, while classed as an Arthropod, it stands very 

 nearly half-way between the ringed worms (Fig. 52 (i)) and the 

 true Arthropoda, and thus forms a solitary link between the two 

 types. In Fig. 52 (2) and Figs. 53 to 58 are shown a number of 

 types of Arthropods. 



We must now go back and take up the main thread of our 

 story. The next stage that falls to be described is that of a 

 highly interesting group of worms known as Enteropneusta, a 

 name signifying ' gut breathers.' This group contains a very 

 small number of worm forms, which are to be found burrowing 

 in the sand of the sea floor. A typical example is the Balano- 

 glossus, a worm of some four inches in length, whose general 

 appearance is illustrated in Fig. 59. The creature has, as will 

 be observed, a large muscular snout or proboscis, behind which 

 follows a small portion called the collar, and behind this again 

 the long body. The most noteworthy feature of the group, as 

 the name implies, is the respiratory system. The mouth, which 

 is situated in the region of the ' collar,' leads into a gullet, which 

 is partially divided into an upper and a lower canal by means of 

 two inwardly projecting longitudinal folds, one on either side. 

 Only the lower of these canals is used as a food passage ; the 



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