SALPA IN RELATION TO EVOLUTION OF LIFE. 207 



The structure of an ordinary nutritive hydranth from a cormus, 

 with its funnel-shaped crown of numerous tentacles placed in a 

 circle around the mouth at the oral end of an elongated body, is 

 undoubtedly an adaptation to a life on the bottom, but there is 

 ample evidence that this hydroid type, while the most familiar one, 

 is not the most primitive, nor the one from which the veiled 

 medusae have been evolved. 



Every one who has reared from the egg many species and indi- 

 viduals knows that the hydra which is formed from the planula, 

 shows a marked tendency to pass through a stage with first four 

 and then eight symmetrical, stiff tentacles, so placed that they do 

 not form a crown, since four point forwards and the other four 

 backwards in such a way that they radiate from the body like the 

 pseudopodia of a pelagic rhizopod. 



In the simplest and most primitive of the hydroids, the Codonidse 

 or tubularias, this larva often has at first, a floating habit, and a 

 distinctive name, actinula, has been given to it on account of its 

 star-like shape. 



The number of tentacles, like the number of arms in a star-fish, 

 varies somewhat, but they are radially arranged in two alternating 

 sets, and in the majority of the individuals there are four primary 

 radii. 



The quadrate structure of young hydroids has been noted by 

 Haeckel, and he points out (Ueber die Individualitat des Thier- 

 korpers, Jena. Zeitschr., XII, 1878, p. 16) that while the number 

 of tentacles may, at first, have been indeterminate and variable, the 

 early establishment of the number four in the organization of the 

 ancestors of the acalephs is shown by the fact that in the young 

 hydra, as well as in many other hydro-polyps, and in the young 

 actinia and in many other acalephs there are only four primary 

 tentacles. 



Haeckel also says that he regards the four-fold condition as primi- 

 tive for the medusae, and that all the six rayed and eight rayed 

 medusae are to be derived from a form with four rays, and that the 

 oral lobes, the primary radial canals and the primary marginal 

 tentacles all lie in the four primary radia. He also gives his reasons 

 for believing that the corals and ctenophores retain traces of this 

 primitive quadrate organization. 



