200 THE HISTORY OF CREATION. 



vol. i. p. 303.) In all Vertebrate animals without exception, 

 man included, these important parts of the body during 

 the embryological development out of the egg, originally 

 begin in the same simple form, which is retained throughout 

 life by the Amphioxus. It is only at a later period that the 

 brain develops by the expansion of the fore end of the spinal 

 marrow, and out of the spinal rod the skull which encloses 

 the brain. As these two important organs do not develop 

 at all in the Amphioxus, we may justly call the class repre- 

 sented by it. Skull-less animals (Acrania), in opposition to 

 all the others, namely, to the animals tvith skulls (Craniota). 

 The Skull-less animals are generally called tubular-hearted 

 (Leptocardia), because a centralized heart does not as yet 

 exist, and the blood is circulated in the body by the con- 

 tractions of the tubular blood-vessels themselves. The 

 Skulled animals, which possess a centralized, thick-waUed, 

 bulb-shaped heart, ought then by way of contrast to be 

 called hulbular-hearted animals (Pachycardia). 



Animals with skulls and central hearts evidently developed 

 gradually in the later primordial period out of those without 

 skulls and with tubular hearts. Of this the ontogeny of 

 skulled animals leaves no doubt. But whence are these 

 same skull-less animals derived ? It is only very lately that 

 an exceedingly surprising answer has been given to this 

 important question. From Kowalewsky's investigations, 

 published in 1867, on the individual development of the 

 Amphioxus and the adhering Sea-squirts (Ascidia) belonging 

 to the class of mantled animals (Tunicata), it has been proved 

 that the ontogenies of these two entirely different looking 

 animal-forms agree in the first stage of development in a 

 most remarkable manner. The freely swimming larvse of the 



