PROTO-CHORDATES 



229 



Branchiostoma (Amphioxus) lanceolatum, a small translucent fish-like 

 marine creature about 2 in. in length the body of which is divided into 

 62 myotomes (Fig. 236). Although possessing no definitive eyes, the 

 animal is strongly photo -negative and sensory organs occur, some 

 possibly in the surface ectoderm and others deeply placed in relation 

 to the neural tube which tend to enforce upon the animal its burrowing 

 habit. 



Fig. 236. — The Lancelet, Amphioxus. 

 The head end is towards the right, the tail end to the left (after Haeckel). 



The superficial sensory organs are the large isolated cells of Joseph 

 (1904-28), associated with the surface eiaitheliuni on the dorsal aspect, which 

 were claimed by this investigator to be light-sensitive (Fig. 237) ; this view, 

 however, is by no means substantiated. 



The neural photosensitive organs are of two types (Fig. 237). Towards the 

 cephalic end of the animal a small median area of ependymal cells lining the 

 central canal of the nerve-cord is specially differentiated to form an infundi- 

 bular ORGAN which appears to be light-sensitive and is functionally allied to a 



Fig. 237. — Sagittal Section of the Anterior Portion of Amphioxus. 



CJ, cells of Joseph ; Inf, infundibulum ; NC, neural oanal ; PS, anterior 

 pigment spot (after Boeke). 



dark pigment -spot situated at the head end of the animal. The pigment-spot 

 was originally described as an " eye-spot " by Johannes Miiller in 1842, and 

 used to be credited with light-sensitive properties and specific connections with 

 the central nervous system ^; it was indeed held to be the phylogenetic precursor 

 of the vertebrate eye. Its specific innervation, however, was contested initially 

 by Kohl (1890) and conclusively by Fi-anz (1923), and a visual function excluded 



1 See the writings of W. Miiller (1874), Langerhans (1876), Ayers (1890), Joseph 

 (1904-28), Edinger (1906), Boeke (1908), Pietschmann (1929). 



