ACRANIA AND CRANIOTA. 417 



equally soft, and were, therefore, equally incapable of being 

 petrified and of leaving any fossil impressions. 



Contrasted with the Skull-less Animals stands the other 

 main division of Vertebrates, embracing all the rest of that 

 class from Fishes to Man. These all have a head, clearly 

 marked from the trunk, with a skull and brain; they all 

 have a centralized heart, developed kidneys, etc. They are 

 called Skulled Animals, or Craniota. But in the earliest 

 stages of their existence even these are skull-less. As we have 

 seen in the Ontogeny of Man, every Mammal, in the early 

 stages of individual development passes through a condition 

 in which it has neither head, nor skull, nor brain, and 

 possesses only the well-known, simple form of a lyre-shaped 

 disc, or of a shoe-sole, without any limbs or extremities. 

 Comparing these early embryonic forms with the developed 

 Lancelet, we may say, that the Amphioxus is in a certain 

 sense a persistent embryo, a permanent germ-form of 

 Skulled-animals ; it never passes beyond a certain low, 

 early youthful condition, out of which we have long since 

 passed. 



The perfectly formed Lancelet (Fig. 151) is 5 to 6 cm. in 

 length (above two inches), is either colourless or slightly 

 reddish, and is shaped like a narrow lanceolate leaf. The 

 body is pointed at both ends, and much compressed later- 

 ally. There is no trace of limbs. The outer skin-covering 

 is very delicate and thin, naked, translucent, and consists of 

 two distinct strata ; a simple external skin, the outer skin 

 (epidermis ; Plate X. Fig. 13, h), and a fibrous leather-skin 

 (corium), lying below the epidermis (Fig. 13, 1). The central 

 line of the back is traversed by a narrow fin-like ridge 

 which widens behind into an oval tail-fin. and is prolonged 



