4-28 THE EVOLUTION OF MAN. 



Fishes. They stand, however, far below the true Fishes, and 

 form a very interesting connecting group between them 

 and the Lancelet. How near they stand to the latter, is 

 clearly seen if an immature Lamprey (Petromyzon, Plate XL 

 Fig. 16) is compared with the Amphioxus (Fig. 15). In 

 both, the notochord (di) is in the same simple form, as is 

 also the medullary tube (m), lying above the notochord, and 

 the intestinal tube (d}, lying below the notochord. But in 

 the Lamprey, the medullary tube soon swells in front into 

 a simple pear-shaped brain-bladder (m^, and on each side 

 of this appears a very simple eye (an) and a simple ear- 

 vesicle (#). The nose (n) is still a single pit, as in the 

 Amphioxus. The two sections of the intestine also, the 

 anterior gill-intestine (&) and the posterior stomach-intes- 

 tine (d), are very simple in the Lamprey, and very like 

 those of the Amphioxus. On the other hand, there is 

 decided progress in the organization of the heart, which 

 appears below the gills as a centralized muscular pouch, and 

 separates into an auricle (hv) and a ventricle (A&). At a 

 later period, the Lamprey attains to a considerably higher 

 state of development, acquires a skull, five brain-bladders, 

 a series of independent gill-pouches, etc. But this makes 

 the remarkable similarity of its young larva to the de- 

 veloped Amphioxus all the more interesting. 116 



The Amphioxus, which is thus directly connected, on 

 the one side, with the Fishes through the Round-Mouths 

 (Cyclostomi), and thereby to the series of higher Vertebrates, 

 is, on the other hand, very nearly allied to a lower inver- 

 tebrate sea-animal, from which, at first sight, it seems very 

 far removed. This remarkable animal is the Sea-squirt, or 

 Ascidian, which until very recently was regarded as being 



