CHAPTER XL 



WORMS. 



This title is justifiable only as a popular name for a shape. 

 The animals included under it form a heterogeneous mob 

 with little in common. There is no class of " worms," but a 

 collection of classes whose relationships are very imperfectly 

 discerned. But if we understand this, there can be no harm 

 in using the title, which is certainly convenient. 



We have seen that the Coelenterates present radial modi- 

 fications of the primitive gastrula-type ; many " worms " are 

 longitudinal and bilateral modifications of the same. The 

 gastrula is elongated, however, not in the direction of the 

 original long axis, but at right angles to it. 



We have also noticed that "worms" were the first multi- 

 cellular animals to move head-foremost. They began to move 

 in one direction, acquiring head and sides. M6reover, as 

 one end constantly experienced the first impressions of 

 external objects, sensitive and nervous cells would tend to 

 be most developed in that " head " region. Thus a brain 

 arose. The nerve-cords, which are lateral in the simpler forms, 

 are combined along the mid-ventral line in the higher. 



Again it may be noted that worms begin the series of 

 Ccelomate animals. In other words, they are the first to 

 acquire a body-cavity or coelome, and a definite middle layer 

 or mesoderm. 



It is not at present possible to have much confidence in 

 preferring one arrangement of the many classes of worms to 

 another, but the following order will be observed here. 

 It is at least certain that the number of great divisions 

 cannot at present be more reduced without giving a false 

 simplicity to the facts. 



