III. GENERAL STUDY OF TYPICAL CHORDATES 



A. AMPHIOXUS 



i. External anatomy of Amphioxus. — Obtain a specimen and place in a 

 dish of water. The body is slender, fishlike, pointed at each end, and compressed 

 laterally. The more blunt end is anterior, the more pointed end, posterior; 

 the dorsal surface is sharp, the ventral surface, for the greater part of its length, 

 flattened. The anterior end represents a poorly developed, somewhat degenerate 

 head. The ventral and greater part of the head consists of an expanded mem- 

 brane, the oral hood, which incloses a cavity, the stomodaeum or vestibule, at the 

 bottom of which the mouth is located. The borders of the oral hood are extended 

 into a series of stiff tentacles or cirri. 



Turn the animal ventral side up and observe that the flattened portion of 

 the ventral surface is bounded laterally by two membranous folds, the meta- 

 pleural folds, or lateral fins, extending posteriorly from the oral hood. These 

 folds meet at a point nearly three-fourths of the distance from anterior to pos- 

 terior end, behind a median opening, the atriopore. From this point a median 

 membranous fold, the fin, passes to the posterior end of the body, around to 

 the dorsal side, and forward along the dorsal side to the anterior end. The 

 slightly wider portion of this fin which surrounds the pointed posterior end is 

 the caudal fin, that along the dorsal side, the dorsal fin. The anal opening will 

 be found on the ventral side very near the posterior end, just behind the point 

 where the fin widens. The anus is on the left side. 



The body is covered by a thin epidermis under which is a muscle layer. The 

 greater part of the muscle layer consists of the lateral muscles, forming the 

 side walls of the body, and divided into a large number of V-shaped muscle 

 segments, or myotomes, clearly visible through the transparent epidermis. Each 

 myotome is separated from its neighbor by a connective tissue partition, the 

 myocomma. Note that the myotomes extend nearly to the tip of the anterior 

 end, (diminishing in size above the oral hood. The number of myotomes in 

 Amphioxus is definite, about sixty. The myotomes are one expression of the 

 metamerism of the Amphioxus body. The ventral portion of the body is 

 provided with a thin layer of transverse or veniral muscles, whose fibers run 

 circularly; these muscles are not visible externally. 



Immediately below the ventral terminations of the myotomes will be seen, 

 in some individuals at least, a row of square white masses, the gonads or reproduc- 

 tive organs. Their arrangement is metameric. The ventral part of the body, 

 extending posteriorly from the oral hood, appears somewhat clear and is occupied 

 by a large cavity, the atrium, which surrounds the digestive tract and opens to 



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