CHORDATA 



313 



lacks paired fins, but has a median dorsal 

 fin which continues over the tail and for- 

 ward a part way on the ventral side. In 

 front of this is a thick fold (metapleure) on 

 either side the body, at the junction of the 

 side with the belly of the animal. The 

 metapleure is thought by some zoologists 

 to be the forerunner of the vertebrate ap- 

 pendages. Amphioocus is without a definite 

 brain ; that is to say, the anterior end of the 

 nervous tube is not highly specialized. It 

 has no skull, eyes, nor ears, such as charac- 

 terize the head of true vertebrates. Amphi- 

 oxus is a small semi-transparent animal 

 about two inches long (Fig. 154). It bur- 

 rows in the sand with only the fringed mouth 

 exposed. It may vary this by swimming 

 about for short periods. 



4. Cyclostomes (Lampreys). These are 

 eel-like animals usually classed with the 

 fishes, and are doubtless more closely related 

 to them than to the forms before mentioned, 

 but because of their primitive qualities they 

 may be placed for the purposes of this course 



FIG. 154. Diagram of the anatomy of Amphioxus, drawn as a 

 semi-transparent object (after Perrier "Traiti de Zoologie"). a, 

 anus; a. p., atrial pore; c.f., caudal fin; dr., cirri, on the edge of the 

 vestibule leading to the mouth; d.f., dorsal fin; r, fin rays; g, gill 

 or branchial structures consisting of alternate slits, through which 

 the water passes, and supporting plates, in the walls of which are the 

 blood vessels; in., intestine, from which as a diverticulum springs 

 /., the liver; m, the mouth surrounded by a fringed velum; my., 

 myotomes or muscle segments; n.c., notochord; o., ovaries; s.c., 

 spinal cord; v.f., ventral fin. 



Questions on the figure. What elements of struc- 

 ture appear in the figure suggesting the chordate charac- 

 ter of Amphioxus? What is the relation of the spinal 

 cord, notochord and digestive tract ? How much of the 

 length of Amphioxus possesses gills? What is the posi- 

 tion, extent and function of the atrium? (Refer to more 

 extended texts.) What structures show evidences of 

 segmentation? What fins has Amphioxus? Compare 

 with fins of fishes. 



FIG. 154. 





