:v/ 



it does not originate a^ a pourii-sliapcd i)rojei-(ing part, hut tliis is 

 inipossil^le, l)ecanse in llie place wlierc tlic nioulli of the ^oung larva 

 will open, the pharynx lies already directly against the e|>iderniis. 

 Neither is a ciliated gill-epilheliuni tbiined in this place, but one 

 has no right to expect it hei-e, because the function of a mouth is 

 so entirely different from that of a gill-slit. On the other hand 

 the month possesses another lasting distinctive pr()|)erty, which is 

 peculiar to every gill-slit during the period of growth, but disappears 

 from these slits in the course of the metamorphosis. I found namely, 

 that each gill-slit of the young larva is accompanied in front and 

 behind l)y a strong gill-muscle, ihe tibres of which, for the greater 

 part, run in a transversal direction with regard to the axis of the 

 body. Some tibres however, surround the outside gill-opening and 

 form a sphincter rouud it. 



The mouth-opening is likewise enclosed by two such muscles. 

 They degenerate also, but they are not lost without leaving a trace, 

 as the gill-muscles proper, but ])roduce the lip-muscJes and the 

 ring-shaped sphincter of the velum. 



Mouth and club-shaped gland are counterparts, for they originate 

 one under the second myotome of the left-side, the other under the 

 second myotome of the right-side of the body. 



In vertebrates the first pair of gill-slits originates nearly under 

 the second myotome, of which in the head of Selachians nine are 

 formed, as I demonstrated more than 30 years ago for the genera 

 Scyllium and Pristuriiis '). In Selachians the first gill-slit does not any 

 longer function as such eilhor; in ravs it serves to admit instead of 

 to let out the respiration-water, and in some sharks this slit, known 

 by the name of spiracle, is shut by the fusion of its parietes. The 

 mouth of Amphioxus is, according to what has just been seen, 

 honn)logons with the left spiracle of Selachians, and serves, just as 

 in rays, to ingest the resj)iration-water, but this \vater contains here 

 the necessary nutriment for the animal. 



If now the mouth of the Anijjliioxus-larva was originally the first 

 gill-slit, then a primitive mouth, homologous with that of vertebrates, 

 must ha\e been extant before this secondary mouth. 



This primiti\e mouth is, in my opinion, represented by the opening 



1) Braus pretends that not 9 bul at least 11 sliould be formed. A repealed 

 invesligalioii, wliicli will l}e pul)lislie(l aflcrwards, lias lauglil me thai my number 

 9 for Scyllium and Prislurius is corrccl, and may be admitted as the normal one 

 for Selachians. In some genera liowevcr vertebrate elements fuse secondarily with 

 the skull. 



