ORIGIN AND ST RUCTURE OF THE HEAD 31 



That the eyes also of Arthropods, which in their situation 

 and their relation to the cerebral ganglia so closely agree 

 with those of worms, are to be derived from the same 

 starting point cannot be doubted. 



Absence of paired eyes in Acrania. — Now in Amphioxus we 

 should expect to find a pair of eyes on the surface of the 

 prostomium, in front of the neuropore, corresponding to 

 those of Annelids. Neither eyes, however, nor cerebral 

 ganglia are to be found here; evidently we must assume 

 that they have been lost as a result of the change of 

 function of the snout, which in Amphioxus serves- for 

 burrowing into the sand while in Ascidians, according to 

 WiLLEY (1894), it is the same organ that serves for attachment. 

 Optic pits in Craniates. — If in Craniates, however, the 

 praechordal region of the body may be compared to the 



praeoral lobe in Annelids, 

 ' ^*- as suggested above, we 



^ * V meet here in very early 



ontogenetic stages a dispo- 

 sition which strongly re- 

 minds us of what is found 

 '^'^ in Annelids. In the devel- 

 opment of Craniates the 

 "^ first indication of the retinal 



area is often noticed already 

 Fig. 8. Rostrolateral view of the .. ..,, . r, . 



head of an embryo of Sus scrofa, ^" ^V }\ ^ P , , ? 

 with 10 mesoderm segments. praechordal cerebral plate 

 From Hertwig's Handbuch T II, as a pair of shallow de- 

 2, p. 156, afteramodellofKEiBEL. pressions of the thickened 

 ectoderm. As well in Elasmobranchs and Amphibians as 

 also in Mammals the occurrence of these optic pits has 

 been known for a long time. The cerebral plate with the 

 eye-pits, situated in front of the provisory neuropore, 

 reminds us strongly of the apical plate of the trochophora 

 with the rudiments of the eyes situated in front of the 

 mouth, especially in such cases where the medullary plate 

 has closed while the cerebral plate is still open, and where, 

 accordingly, the anterior opening of the former represents 

 a kind of provisory neuropore corresponding to that of 

 Amphioxus and, if our conclusions until now are right, to 

 the mouth of Annelids. An example of such a case is given 

 by fig. 8, after KEIBEL. Especially interesting in this respect 

 are the observations of EyclesHEIMER (1893, 1895) on 



