430 THE CIRRIPEDS, TUNICATES AND ECHINODERMS. 



words, a negative character at one point can create a new character at a dif- 

 ferent point. 



Echinoderms and Cirripeds. Summary. 



1. The echinoderms are descended from cirriped-like arthropods in which, 

 as a result of some unknown condition, the absence, or degeneration of organs 

 on one side of the middle section of the body had become a frequent or fixed 

 character. 



The five half metameres corresponding to the five defective ones, assumed 

 in consequence, a new architectural arrangement, forming a closed ring with the 

 segmental organs arranged in radiating lines, instead of a double linear series of 

 parallel lines. The five half metameres successfully established a new condition 

 of organic stability, and gave rise to a new kind of body, and a new class of animals; 

 the bilaterally symmetrical head and tail ends of the old body either atrophied 

 completely, or were reduced to structural insignificance. 



2. The relation of the echinoderms to the arthropods is shown by the absence, 

 or imperfect repetition of a true gastrula stage; by the conspicuous development 

 of a telopore and telocoele; by the development of a nauplius-like larval form, or 

 naupula; and by the mode of development of the mesoderm, mesoblastic somites, 

 coelom, and excretory ducts. 



3. The echinoderms more particularly resemble the cirripeds in the form 

 of the larvae, and in their mode of attachment and rotation; also in the mode of 

 growth of the mantle, atrial chamber, appendages, and cephalic stalk; and in the 

 peculiar sessile mode of life that is so characteristic of practically all the more 

 primitive members of the class. 



