4o6 THE LANCEI.KT '""Ai'. 



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

 corpuscles. 'I'he nephridia reaiain distinct, not being 

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

 anteriorly, in the neighbourhood of the pharynx (Fig. 99, 

 /;///) ; the gonads (Figs. 98 and 99, goit, and Fig. 100 A, .1,'') 

 are metamerically arranged, and have no ducts. 



Ill certain of its cliaracters Amphioxus resembles the members of 

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

 in \\hich the boih' is enclosed in a " lest" or mantle, consisting largely 

 of cellulose (p. 244). These, likeA'ertebrates, 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 \'ertebrates 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 

 I'ig. 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. lid), from which a number of tentacles or cirri (cir) are 

 gl\en off; and that there is a lateral ox iiic/aplt'iiral fo/d (//itpf) 

 along either side of the body extending ba(.'kwards as far as 

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

 I'd. f, vent. J) extending round the tail as a caudal fin. In 

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

 but a median <'anal is subsequently formed along the 

 viiiiral side of the body, and as this extends inwards to 

 form the atrium, it graduall)' surrounds the pharynx at the 

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

 liceomes greatly reduced in this region (Fig. 100 .\, eo). 

 The ih-liimi is \iiy simple ; besidi's ihr noloehord there 



