CHAPTER XVI. THE LANCELET. 

 (BrancMostoma lanceolatum or Am2)hioxus lanceolatus.) 



1. External Characters. The Amphioxus is a small 

 fish-like animal found in moderately deep water round the 

 sea-coasts in many parts of the world. It is usually about 

 two inches in length, sharply pointed at both ends, and 

 in the living state is almost transparent. Myomeres, 

 V-shaped as in the dogfish and other fishes, about sixty-two 

 in number, show through the thin skin and emphasize the 

 Vertebrate relationship of the animal; but these extend 

 forwards right to the anterior end of the body, and there 

 is no well-defined head as in all higher Vertebrates. With 



OVCCAL C//?/?/ 



Fig. 102. AMPHIOXUS. 

 (After Lankester.) For " Myotome " read (preferably) " Myomere." 







the absence of a head goes the absence of all elaborate 

 sense-organs eyes, ears, olfactory sacs. Gill-slits are pre- 

 sent, and very numerous, but hidden from view, as we shall 

 presently see. Instead of a series of median fins, we find 

 a continuous median fin extending along the dorsal edge 

 (for the Amphioxus is even more compressed laterally than 

 the dogfish) from snout to tail, slightly expanded at the 

 hind end into a caudal fin, and then continuing forwards 

 on the ventral side beyond the anus, which is shifted by 

 it slightly to the left. Such a continuous median fin, and 

 such displacement of the anus, are also found in some of 

 the true fishes. This median fin extends forwards to the 

 thirby-sixth myomere (counting from before backwards) and 



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