THE ECHINODERMS. 421 



anterior haemal surface of the head. On the other hand, in the cirripeds the gut 

 pouches may have an arrangement in transverse rows similar to that in the tuni- 

 cates; the old stomodaeum often closes up and ceases to function, while the cephalic 

 navel may form a temporary opening into the enteron, in the place where the new 

 tunicate mouth is formed; and in the parasitic cirripeds the region of the cephalic 

 navel may actually serve for the absorption of nutrition. 



Thus many important conditions essential to the evolution of tunicates are 

 present in cirripeds, and on the whole the present structure of the tunicates is 

 more like that of cirripeds than that of any other known animals. 



III. The Echinoderms. 



The echinoderms must be assigned a position in our scheme because they 

 appear to be in some way connected with the enteropneusta, and hence with the 

 tunicates and with the main phylum from which the chordates arose. The prob- 

 lem is a difficult one. It demands some explanation of the apparent resemblance 

 between echinoderms, enteropneusta, and tunicates, and if there is a real resem- 

 blance, indicating genetic relationship, it calls on us to harmonize the probable 

 origin of the echinoderms with the explanation we have given above for the origin 

 of the other chordate phyla. The key that unlocks this series of problems and 

 places at our disposal a consistent and ever ready explanation of the multitude of 

 details involved, must indeed be the master key. 



The echinoderms are notable for their contrasts and contradictions. Their 

 outward appearance and their pronounced radial structure distinguish them from 

 all other animals, and at first sight suggest a very primitive organization similar to 

 that of the coelenterates. On the other hand, they display a high degree of histo- 

 logical and anatomical specialization that is in marked contrast with their low 

 grade of organic efficiency. They begin their early embryonic development with 

 a bilaterally symmetrical body and with clear indications of metamerism, only to 

 change it in the later stages for one that is radially symmetrical, and in which all 

 outward traces of metamerism have disappeared. After a short free swimming 

 larval existence they attach themselves, neural side down, by means of larval 

 appendages and a cephalic outgrowth; they then turn neural side up, and remain 

 so attached for life; or, in some cases, they give up their sessile existence and again 

 become free, moving slowly about, neural side down. 



There are, therefore, three chief characteristics of the echinoderms that 

 demand our first consideration: The early, bilateral symmetry and metamerism; 

 the sessile life and mode of attachment by cephalic outgrowths; and the asymmetry. 

 There appears to be but one explanation for these remarkable conditions, which 

 is as follows: The early development of bilateral symmetry and metamerism in 

 the echinoderms, and the presence of a telocoele and telopore in place of the more 

 primitive gastrula and blastopore, clearly indicate that they had their origin in 

 bilaterallv symmetrical animals of the acraniate type, that had already acquired a 



