SALPA IN RELATION TO EVOLUTION OF LIFE. 201 



that they are formed in succession from in front backwards, as 

 transverse slits without any trace of a U-shaped fold. Salensky 

 has more recently (Zool. Jahrbiicher, IV, 1891) studied their 

 origin in the primary assidiozooids of pyrosoma, by sections, and 

 has confirmed Seeliger's statement that each slit is an independent 

 perforation. We have pretty satisfactory evidence that this is true 

 of doliolum also, and Barrois' observations indicate that it is true 

 of anchinia. 



The supposed resemblance between the first pair of gill-slits of 

 ciona and a single horseshoe-shaped slit with a tongue bar, com- 

 parable to the gill-slits of amphioxus is not based upon direct 

 observation, however, for while Willey tells us in his summary, 

 p. 353, that " The four primary stigmata of Ciona intestinalis are 

 developed from one primitive gill-slit," we find, on page 322, 

 that " in the actual ontogeny " the two primary gill-slits " arise by 

 independent perforations." 



The Origin of the Craspedota. 



This section loas intended, ivhen written, to be part of my memoir 

 on the genus Salpa, bid lack of room prevented its publication. 



Another group of animals, which, notwithstanding the text- 

 books, I regard as a product of pelagic influences, is the craspedota, 

 or so called hydro-medusae. 



The opinion that the hydroid jelly-fish is one of the polymor- 

 phic members of a hydroid-cormus, is, like the prevailing views 

 on the origin of the chordata and arthropods, a result from the 

 dogma that the aggregation of units into a compound whole 

 must necessarily be earlier in time than the high evolution of 

 the units. 



The blastostyles and machopolyps of hydroids are, unquestion- 

 ably, modified hydranths, which have arisen, in a cormus, by di- 

 vision of labor, and our first impulse is to believe that the origin 

 of tiie sexual medusae must have been the same. 



The persistency of this opinion is natural. There seem to be 

 many proofs that the remote ancestors of the hydro-medusae were 

 sessile, but I shall try to show that none of them are valid. 



