EXTERNAL FORM. 13 



such a way as to form two sides of a triangle, the apex 

 of which is directed forwards. These are the partitions 

 or septa which divide the longitudinal muscles of the 

 body into a series of separate muscle-chambers or vivu- 

 touics. In virtue of the longitudinal muscles being broken 

 up, so to speak, into a great number of segments, the 

 animal is enabled to swim rapidly with a serpentine 

 motion. In the remarkable pelagic animal, Sagitta, where 

 the muscles are not segmented, this motion is impossible, 

 and instead, it darts forward by sudden and spasmodic 

 jerkings of its tail. 



In Amphioxus, the tail or post-anal region of the body 

 is very much reduced, and the muscle-segments of the 

 trunk therefore constitute its only means of locomotion, 

 there being no muscular fins. Beyond the muscle-plates, 

 both in front and behind, the notocJiord, which forms the 

 axial skeleton of the body, is seen to extend to the anterior 

 and posterior extremities. The extension of the notochord 

 beyond the anterior limit of the dorsal nerve-tube is a very 

 exceptional condition, and has led to the creation of a 

 special order for the reception of Amphioxus ; namely, the 

 Cephalochorda. 



The oval structures seen lying below the muscle-plates 

 in Fig. i are the reproductive organs, male or female as 

 the case may be. Instead of being represented by a single 

 genital gland on each side of the body as they are in the 

 higher fishes and Vertebrates generally, they consist here 

 of some twenty-six pairs of perfectly distinct chambers, 

 occurring in correspondence with the muscle-segments or 

 myotomes of the region to which they belong, and extend- 

 ing from the tenth to the thirty-fifth myotome inclusive. 

 These chambers are known as the gonadic poucJics. (See 

 Fig. 2.) 



