150 



AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



Fig. 42. THREE LARVAL STAGES or 

 AMPHIOXUS (from Glaus, after 

 Hatschek). D, Stage with 2 pairs 

 of mesodermic somites, seen in opti- 

 cal longitudinal section, dorsal side 

 to right. E, Stage with 9 pairs of 

 mesodermic somites, seen from above. 

 F, Living larva with mouth and first 

 gill-slit, seen from the left side ; the 

 2nd, 4th, and 6th bent lines represent 

 respectively the posterior boundary 

 of the 1st, 2nd, and 3rd somite of 

 the opposite side. Bl, ventral blood- 

 vessel ; Gh t notochord ; D, intestine ; 

 K, gill-slit ; MF, unsegmented meso- 

 derm fold, behind which one teloblast 

 is seen in D, both in E ; N, neural 

 canal ; 0, mouth ; Oc, anterior open- 

 ing of neural canal ; Us, mesodermic 

 somites. 



head (see Fig. 39), but por- 

 tions of it are converted into- 

 the fin-ray lymph-spaces and 

 the metapleural canals. The 

 ventral sections of the somites 

 grow downwards, ultimately 

 fusing below the gut, the 

 wall of which, outside the- 

 epithelium, is formed from 

 their splanchnic layer. The 

 splanchnocoele becomes the 

 coelom, which is later on 

 split up into sections. A 

 body-cavity, which, like this, 

 is derived by the outgrowth 

 of myocoelomic pouches, is 

 said to be enterocoelic, and 

 may be called an enteroc&h. 

 Such a simple mode of origin 

 is probably a very primitive 

 one. 



The anterior unpaired arch- 

 enteric pouch divides into 

 right and left halves, of which 

 the latter becomes the pro- 

 blematic sense organ that, in 

 the adult, opens on the roof 

 of the mouth. 



The notochord is developed 

 as a longitudinal fold of the 

 archenteric wall in the mid- 

 dorsal line. 



The central nervous sys- 

 tem arises in a way which, 

 in its more general features, 

 is characteristic of all Chor- 

 dates. The ectoderm cover- 

 ing the flattened dorsal sur- 

 face of the completed gastrula 

 becomes marked off by a 

 slight furrow from the lateral 



