12 PHYLUM CEPHALOCHORDA. 



The anus is also ventral and is placed slightly to the left of 

 the middle line at some little distance from the posterior end 

 of the body (Fig. 5). Extending from the mouth backwards 

 along the ventral surface for about two-thirds of the length of 

 the animal is a wide median groove, bounded by lateral folds 

 and perforated at its hind end by a pore (Fig. 5). The folds are 

 called the metapleural folds and the pore the atrial pore. 



There are no paired fins, but there is a continuous median fin 

 consisting of a fold of skin extending along the whole length of the 

 dorsal surface (dorsal fin), and round the hind end of the body 

 on to the ventral surface as far forwards as the ventral groove 

 (Fig. 4). Anteriorly it is also continued on the ventral surface, 

 reaching as far as the mouth, with the right side of which it is 

 continuous (Fig. 5). The portion between the ventral groove 

 and the anus may be called the anal fin, and that between the 

 anus and the hind end of the body the ventral part of the caudal 

 fin. 



Amfhioxus is a segmented animal. The segmentation is 

 marked externally by a number of V-shaped grooves, placed 

 one behind the other on each side of the body, the apex of the 

 V being directed forwards (Fig. 4). These markings are caused 

 by the insertion into the skin of a number of transverse septa 

 of connective tissue, which divide the great lateral longitudinal 

 muscles of the body into a series of successive segments, placed 

 one behind the other and called myotomes. The grooves of the 

 two sides of the body alternate with one another. The seg- 

 mentation is also exhibited by the gonads which consist of a 

 series of saccular bodies extending throughout the greater part 

 of the pharyngeal region as far back as the atrial pore (Fig. 4). 

 They correspond in number with the myotomes of that part of 

 the body in which they occur and alternate with those of the 

 opposite side of the body. 



The body of Amphioxus is traversed throughout almost 

 its entire length by a flexible skeletal rod — the notochord. 

 The notochord is pointed at either end and is placed in the 

 centre of the body, but nearer to the dorsal than the ventral 

 surface (Fig. 4). Lying immediately on the dorsal side of the 

 notochord is a cord of nervous matter which may be called the 

 cerebrospinal cord and constitutes the central nervous system. 

 Behind, this nervous cord tapers and ends in a point, or a small 



