ix EXTERNAL CHARACTERS 421 



similar relations. There is no heart (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 

 (Figs. 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 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 Verte- 

 brata; 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. 106. 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 me tuple ural fold (mtpl) 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, 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, si, p), leaving 



