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CHAPTER II. 

 THE DEVELOPMENT OF AMPHIOXUS. 



I. GENERAL ACCOUNT. 



1. Structure of Amphioxus. 



Amphioxus is a small, semi-transparent, fish-like animal, 

 about a couple of inches in length, found in shallow parts of the 

 Mediterranean and other seas. It is of sluggish habits, and 

 usually remains buried in the sand, either completely or with 

 the anterior end alone protruding ; but if disturbed it swims 

 actively, by rapid lateral movements of the body. 



In the general plan of its organisation Amphioxus agrees 

 with the more familiar members of the group of Vertebrates, but 

 in a large number of important respects it is far simpler than 

 any of these. 



The external appearance of Amphioxus is shown in Fig. 11. 

 The body is elongated, laterally compressed, and pointed at both 

 ends. There is no distinct head, and no trace of limbs. 



A low dorsal fin runs along the middorsal line from end 

 to end of the animal, becoming more prominent at the hinder 

 end as the upper lobe of the caudal fin. The ventral surface 

 bears a median fin along its posterior third, but in front of this 

 is flattened, so that the body is triangular in section. The sides 

 of this flattened ventral surface are bordered by the lateral fins 

 or metapleural folds. (Gf. Figs. 11, 12, 13.) 



The skeleton is in an extremely simple condition. Neither 

 cartilage nor bone is present, and the principal skeletal structure 

 is an elongated elastic rod, the notochord (Fig. 11, K), which 

 extends the entire length of the animal, lying dorsal to the ali- 

 mentary canal and between this and the spinal cord. The noto- 

 chord is surrounded by a thick sheath of dense connective tissue 

 (Fig. 12, D), which is prolonged dorsalwards to form a tubular 

 investment around the spinal cord. From these sheaths to the 



