ix EXTERNAL CHARACTERS 421 



(Fig. no), and the colourless blood apparently contains no 

 corpuscles of any kind. The nephridia remain distinct, not 

 being united into a single kidney on either side : they are 

 situated anteriorly, in the neighbourhood of the pharynx 

 (Fig. 107, nph, and 109 A, n) ; the gonads (Figs. 106 and 

 107, gon, and Fig. 109 A, g) are metamerically arranged, 

 and have no ducts. 



In certain of its characters Amphioxus resembles the members of 

 a group of animals the Ttmicata, commonly known as "sea-squirts," 

 in which the body is enclosed in a " test " or mantle, consisting largely 

 of cellulose (p. 244). These, like Vertebrates, possess a notochord and a 

 dorsal, hollow nervous system in young stages, and in the adult retain 

 numerous pharyngeal gill-slits ; they are almost certainly degenerate 

 descendants of primitive animals from which the Vertebrata also arose. 



These numerous and marked differences between the 

 lancelet and the higher Vertebrates make it necessary to 

 place Amphioxus in a separate division of the Vertebrata, 

 called from one important negative character the 

 Acrania, while all the other Vertebrates, which possess 

 skulls, are included in the division Craniata. 



The external appearance of Amphioxus is represented in 

 Fig. 1 06. In addition to the points already referred to, it will 

 be seen that the mouth is surrounded by a fold, the oral hood 

 (or. hd\ from which a number of tentacles or cirri (cir) are 

 given off; and that there is a lateral Q\ metapleuralfold(mtpt) 

 along either side of the body extending backwards as far as 

 the atrial pore, in addition to the median fin-fold (dors, f, 

 cd. /, vent, f) extending round the tail as a caudal fin. In 

 the young animal the gill-slits open directly to the exterior, 

 but a median canal is subsequently formed along the 

 ventral side of the body, and as this extends inwards and its 

 edges unite to form the atrium (Fig. 108 s/, p\ leaving only 

 the atriopore open, it gradually surrounds the pharynx at the 



