1006 



animals such as Annelids, Molluscs, Arthropods, Chordates and Ente- 

 ropneusts, as is the case already in Volvox and the pelagic blastulae 

 and larvae. 



Sessile forms such as Coelenterates, Echinoderms, Ascidians (Willey, 

 1894, p. 329), however, often attach themselves with the anterior 

 end. Then the prae-oral lobe loses its significance as a sensory and 

 nervous centre. This is equally the case in burrowing animals like 

 the earth-worm, Amphio.vus and BnJanoglossiis. 



In Polychaetous Annelids the whole segmented soma, ecto-, ento- 

 as well as mesoderm, takes its origin from the four vegetative cells 

 of the eight-celled stage, i.e. fiom the vegetative half of the blastula. 

 Prostomium and soma are met with again in Chordates. With 

 Annelids and Anthropods one or some of the anterior segments 

 unite with the prostomium to form the head; this is equally the 

 case in Vertebrates. 



In Ampkioxus *) we can hardly speak of anything of the kind. 

 The Annelidan stomodaeum has grown out in a backward direction 

 and has become the medullary tube (Dei.sman, 1913a, p. 649), which 

 even surpasses the soma in length (formation of the tail, Delsman, 

 1917b, p. 1271). The mouth, situated in Annelids ventrally just behind 

 the limit of prostomium and tirst segment (peristomium), is found 

 again in Amphioxus as the neuropore on the corresponding place 

 (Delsman, 1913b), viz. dorsally, at the boundary of prostomium and 

 soma, just in front of the first mesodermic segment, which is the man- 

 dibular segment of van Wijhe (1893, [). 157), the "collar-cavity" of 

 MacBrtde (1898, p. 599). The fore-end of the notochord is originally 

 situated right under the neuropore and equally indicates the limit 

 of prostomium and soma. Sense organs and ganglia have been 

 lost or become indistinct in Ampldoxns. The brain vesicle cor- 

 responds to the deuterencephalon (Kupffer, 1905j of Craniotes. 

 The somites from the foremost up to the last develop uniform 

 myotomes constituting together the voluntary longitudinal trunk 

 musculature. The first pair sends out a "rostral prolongation" 

 (Kopffortsatz) into the prae-oral lobe in which also muscle fibres 

 develop which, however, subsequently disappear. 



In the larva the gill-clefts regularly alternate with the myotomes 

 (Willey's figures, 1891, Hatschek, 1892, p. 145), so there is eume- 

 tamerism. Only secondarily, after the "critical stage" (Willey, 1891, 

 p. 202), it gets lost. The left gill-cleft between the first and the 

 second somite becomes the larval mouth, its antimere is the club- 



1) The reader is invited to compare the following descriptions with the plate. 



