PHYLUM CHORDATA 



'I'ho phylum Chordata, which is the last in tho scries, comprises 

 animals which are diverse in form and structure. All are alike, 

 however, in the possession of certain fundamental characteristics 

 of organization which are not found combined in any other 

 animals. The phylum is divisible into four subphyla, a represent- 

 ative of one of which will be studies by way of gathering first- 

 hand information regarding the general plan of organization 

 which prevails throughout the phylum. 



Subphylum Cephalochorda Branchiostoma (Amphioxus). — 

 This subphylum comprises about a dozen species of small marine 

 animals called "lancelets." They possess in a typical state the 

 fundamentals of organization common to all chordates. These 

 animals are found near the shore, where they burrow in the sand 

 which may be entered either head or tail foremost. The body, 

 with the exception of the anterior end, is usually concealed by 

 day, but the animals may leave their burrows at night and dart 

 through the water. While rapid and powerful swimmers, they 

 topple over upon their sides when forward progression ceases. 



External Structure. — Study a stained and mounted specimen 

 of a young animal under the dissecting microscope or low power 

 of the compound microscope and note the spindle-shaped body 

 and the lack of a well-defined head. Along the middorsal line 

 note the low, nearly transparent fold extending the entire length 

 of the body. This fold is the dorsal fin which becomes broader 

 in the tail region and is continued around the end of the tail 

 to the ventral side where it narrows in its cephalic extent. This 

 broader portion of the fin fold about the end of the tail is known 

 as the caudal fin, while the narrower ventral part extending 

 cephalad from the caudal is the anal fm. Two ventro-lateral 

 folds, the metapleural folds, border the flattened ventral surface 

 of the anterior two-thirds of the body and give this portion a 

 triangular shape when viewed in cross-section. These may be 

 seen in preserved specimens. 



The sides of the body are marked by a series of V-shaped lines, 

 formed by niyo.septa of connective tissue which divide the great 



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