EXTERNAL ANATOMY OF THE CHORD ATES 1 



1. Examine the worm-like Chordate, Dolichoglossus, and note the 

 following structural features: (a) the blunt hollow proboscis at the 

 anterior end, just back of which is (b) the collar encircling the anterior 

 end of the body. Posterior to the collar on the dorsal side of the body 

 is (c) a double row of small openings, the gill slits, which are believed 

 to indicate chordate relationship. Internally Dolichoglossus possesses a 

 notochord-like structure and a dorsal nervous system, both of which are 

 also generally regarded as additional evidence of Chordate relationship. 

 On the whole it must be emphasized that the taxonomic position of this 

 species is open to considerable question. Draw to show the external 

 structures. 



2. Examine one of the so-called Sea-squirts, such as Molgula. In 

 the adult condition these animals are attached to some solid object and 

 show no external characters which would link them with the typical 

 Chordate. The rather cylindrical body is completely enclosed in (a) a 

 tunic largely composed of cellulose. Note (b) the oral aperture and 

 (c) the atrial aperture. During life a current of water is continually 

 drawn into the oral aperture, then passes through (d) the inner branchial 

 chamber, the walls of which contain numerous ciliated gill slits. Next 

 the water passes into the surrounding atrial cavity and then out of the 

 body through the atrial aperture. The tiny, microscopic larval stage of 

 this group is a free-swimming tadpole-like organism which unquestion- 

 ably possesses the three basic chordate features. Examine a preparation 

 of the larval stage if available and note the general structural arrange- 

 ments. Draw both stages in outline to show structures observed. 



3. Examine the small free-swimming Chordate, Branchiostoma, more 

 commonly known as Amphioxus. Note that the elongated body is 

 definitely fish-like in its general shape. Identify the anterior, posterior, 

 dorsal, and ventral surfaces. Note: (a) the opening (oral hood) at 

 the extreme anterior end of the body surrounded by (6) a fringe of cirri 

 and opening into (c) a cavity (vestibule) at the base of which is (d) the 

 jawless mouth, (e) The dorsal fin will be found extending practically 

 the length of the body on the dorsal side and joining posteriorly with 

 (/) the tail (caudal) fin. The latter is continuous with (g) the ventral 

 fin, which extends anteriorly about one-third the length of the body. 

 Note further (h) a marked segmentation (metamerism) of the muscle 

 tissue in the body wall. On the ventral surface locate (i) the anal 

 opening near the anterior end of the caudal fin and also (j) the opening 

 (atriopore) at the posterior end of the ventral (k) metapleural fold 

 which encloses the atrial cavity. Draw in outline to show the structure 

 as observed. 



1 B. pp. 149-152. 

 307 



