44 AMPHIOXUS. 



Th e ova, which measure 0-105 mm. in diameter, appear to 

 es cape from the atrial oa vitv. as a rule, through the atrial pore ; 

 but in some cases they have been seen to pass through the gill- 

 slits into the pharynx, and to make their exit through t he 

 mou th - 

 i n the ma le, the tgstes ar e simila r in form and posit ion to 

 the .Qvaries of th e fem ale ; and the spermatozoa when ripe are 

 discharged, like the ova, into the atrial cavity, from which they 

 escape by the atrial pore. 



2. Morphological Importance of AmpMoxus. 



It wiU be seen from the preceding account of its anatomy 

 that Amphioxus, while clearly and undoubtedly a Vertebrate, yet 

 differs from all ordinary Vertebrates, whether fish, amphibians, 

 reptiles, birds, or mammals, in a number of points which are of 

 great importance and affect almost every part of its body. 



A closer examination shows that these po iiits of differe nce 

 betw een Amphioxus and the higher Vertebrates may be g rouped 



nndCT t,wn rh\p.f hpflrls . 



P ) The atrial cavityi the l ar ge number of the gill-sl its, the 

 regular alternation of gill-arches of two kinds, the azygo s 

 c haracter of the sense organs , the e xtension of the ■nnf.n^'hnTrl in 

 th e extrem e ant erior end of the anima l, and t he curious later al 

 asymmetry show n by the myotom es, nerves, an d other organs , 

 are examples of a group of characters in which Amphioxus 

 differs from the higher Vertebrates, not only in their adult con- 

 dition, but at all stages of their existence. 



(2) T here is another and even more striking series of cha- 

 ratrCSrs^in which Amphioxus differs from the adult forms of the 

 higher Vertebrates, but resembles these in their early develop- 

 mental stages. Thus in all the higher Vertebrates there is a 

 s tage in development whe n the nntnchnrd is the on l y skeleta l 

 structure present , ne ither cartilao-e nor bone having ye tappeared ; 

 a stage in which the lim^bp a.rp. a bsent ; and a stage in whi ch 

 t he jail spIps of the body have th e simple and definite segmental 

 a rrangement seen in Amphioxus throughout life. In ail higher 

 Vertebrates the h ^rt is at first straight, lik e the cardiac aorta 

 of Amphioxus ; tJie 'l iver arises as one or more outgrowths of 

 the intestine ; and the dorsal and ventral roots of the spina l 

 nerves are at first independent of each other. In these and in 



