414 



THE CIRRIPEDS, TUNICATES AND ECHTNODERMS. 



appearance of the stomodaeum. We have already witnessed a tendency in this 

 direction, in the arachnid-vertebrate stock, and the origin of a new mouth, 

 or h^mostoma, from the dorsal organ, or cephalic navel, p. 253. Similar condi- 

 tions are latent in the cirripeds. A typical embryonic dorsal organ is a conspic- 

 uous feature in the young nauplius of Lernaea and of many other primitive crus- 

 tacea. Its invagination, or ingrowth into the yolk, followed by disintegration 

 and absorption, probably played an important part in establishing a permanent 

 opening into the enteron at this point, from which, in the other sub-phyla of the 

 acraniates, the new mouth arose. The exact way in which the new opening 



c.r 



cr. 

 LerncBa. 



FIG. 282. Lernaea branchialis. A, Young fertilized female; B, penella stage; fertilized female in gill of Whiting; C, 

 adult female, attached. (After Scott, slightly modified.) 



was established, and its relation to the adjacent gut pouches, to the adhesive 

 glands, and to the cephalic stalk are not clear because very little is known about 

 the details of these important structures. But it is certainly significant that in the 

 rhizocephala, where the old mouth closes, the animals manage to survive by the 

 absorption of nutrition through the root-like outgrowth of the cephalic stalk that 

 is formed at the place where the dorsal organ closes. The condition in Tubicinella 

 is likewise suggestive, for Gruvel states, p. 279, that according to Marloth, the 

 tubicinellae secrete a peptonizing ferment that diffuses through the membranous 

 base, transforming into peptones the albuminoid substances of the skin of the whale, 

 to which these forms are attached. Without doubt we have here a glimpse of the 

 way in which the old mouth disappeared, and the way in which the new one was 



