4o6 THE LANCELET chap. 



(Fig. loi), and the colourless blood contains no red 

 corpuscles. 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, 99, 

 npK) ; the gonads (Figs. 98 and 99, gon, and Fig. 100 A, g) 

 are metamerically arranged, and have no ducts. 



In certain of its characters Amphioxus resembles the members of 

 a group of animals — the Tunicata, 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. 98. 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 or metaplenral fold (mipl) 

 along either side of the body extending backwards as far as 

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

 cd. f, ve?it. 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 to 

 form the atrium, it gradually surrounds the pharjmx at the 

 sides, pushing the ccelome before it, so that the latter 

 becomes greatly reduced in this region (Fig. 100 A, co). 



The skeleton is very simple : besides the notochord there 



