3io 



ZOOLOGY. 



FIG. 152. 



L-e 



muscles are arranged in segments which 

 appear externally. It lacks paired fins, 

 but has a median dorsal fin which con- 

 tinues over the tail and forward a part 

 way on the ventral side. In front of this 

 is a thick fold (inetaplenre) 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 appendages. 

 Amphioxus is without a definite brain, 

 that is to say, the anterior end of the ner- 

 vous tube is not highly specialized. It 

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

 acterize the head of true vertebrates. 

 Amphioxus is a small semi-transparent 

 animal about two inches long (Fig. 

 152). It burrows in the sand with only 

 the fringed mouth exposed. It may vary 

 this by swimming about for short periods. 



FIG. 152. Diagram of the anatomy of Amphioxus, 

 drawn as a semi-transparent object (after Perrier " Traite 

 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., ven- 

 tral hn. 



Questions on the figure. What elements of 

 structure appear in the -figure suggesting the 

 chordate character of Amphioxus? What is the 

 relation of the spinal cord, notochord and diges- 

 tive tract? How much of the length of Amphi- 

 oxus possesses gills? What is the position, ex- 

 tent and function of the atrium? (Refer to more 

 extended texts.) What structures show evi- 

 dences of segmentation? What fins has Amphi- 

 oxus? Compare with fins of fishes. 



