4 6o THE LANCELETS— CEPHALOCHORDA 



cord and, though skeletal, does not attain to the definiteness of 

 cartilage. 



The gill-slits on the pharynx become very numerous, and 

 each is divided into two by a curious "tongue-bar"; in the 

 adult they open not to the exterior, but into an ingrowing 

 (ectodermic) atrial or peribranchial cavity, which obliterates 

 most of the body-cavity or ccelom in the pharyngeal region. 

 The body-cavity arises as a set of pouches from the primitive 

 gut (enteroccelic mode of origin). 



The muscular system is very markedly segmented, the 

 body-wall showing over fifty myotomes ; the gonads are also 

 segmentally disposed. 



As regards the general scheme of the circulation there is a 

 resemblance to that in fishes, but Lancelets are widely removed 

 from fishes by the absence of vertebrae, limbs, skull, jaws, dif- 

 ferentiated brain, sympathetic nervous system, brain-eye, ear, 

 definite heart, pancreas, spleen, and genital ducts. 



In certain respects, such as the persistently obvious segmen- 

 tation of the whole body, the state of the nerve-cord and noto- 

 chord, the numerous separate nephridia, and the segmental 

 gonads without ducts, the Lancelets are primitive. 



The eggs segment wholly, a typical gastrula is formed by 

 invagination, the nervous system and the notochord arise in a 

 very typical way, the mesoderm arises from mesodermic coelomic 

 pouches. 



The larval forms are strangely asymmetrical and the larval 

 period is prolonged. 



The following conclusions may be drawn concerning the 

 relationships of Lancelets : — 



I. There is no longer any doubt that Amphioxus belongs 

 to the Chordate series. 



It has(i) a dorsal, tubular, spinal cord, (2) a dorsal sup- 

 porting axis or notochord, (3) pharyngeal gill-slits, and (4) a 

 segmented body. We miss altogether the characteristic Chordate 

 eyes (arising as paired outgrowths from the brain) ; we note 

 also the absence of a definite heart (arising from the specialisa- 

 tion of part of a ventral blood-vessel or of two ventral 

 blood-vessels), but there are numerous " hearts," as it were, 

 taking the place of one, and the general course of the circula- 

 tion is vertebrate-like ; there is a total absence of elements 



