90 WILLIAM BATBSON. 



fact does not interfere with the obvious possibility of a digging 

 mouth having again intervened, from which such a mouth as 

 that of the Lampreys could easily be derived. 



Taking into consideration, then, the fact that in the most 

 primitive forms the mouth is anteriorly directed, and that in 

 the Lampreys it is also anteriorly directed, though of different 

 function, we may tentatively suppose that though the mouth of 

 the possibly original pelagic form was directed ventralwards, and 

 was possibly suctorial, yet probably the mouth of the Marsipo- 

 branchsis derived from a digging ancestor, in which the mouth 

 of the hypothetical pelagic form had come to be anteriorly 

 directed in correlation with an acquired burrowing habit. In 

 any case the facts of the Enteropneusta entirely confirm Bal- 

 four's view, that the Vertebrate jaws have been developed com- 

 paratively long afterwards. 



The Skin. — That the skin was originally ciliated there can 

 be little doubt ; also it is probable that at first plexuses of 

 nerve-fibre were formed at the base of the ectoderm cells, such 

 as may be seen in many if not in all animals with ciliated skins 

 of this type. 



The Nervous System. — The next question relates to the 

 position and mode of the first formation of a differentiated ner- 

 vous system. The evidence of Enteropneusta, Ascidians, and 

 Amphioxus is united in showing that this first occurred in the 

 dorsal middle line, and not by the coalescence of two lateral 

 cords. The structure of the nervous system of Balanoglossus 

 farther shows us a stage in the process by which this nervous 

 cord separated from the skin. By many authors it is supposed 

 that this was accomplished in the first Chordata by an invagi- 

 nation, but the evidence of Balanoglossus is decidedly for the 

 view that a process of delamination preceded this ; and, indeed, 

 this being the simple process, might naturally have been ex- 

 pected to have occurred first. In Balanoglossus we see in 

 the trunk the cord still in the skin, in the collar the 

 cord delaminated, and at the ends of this cord the 

 process of invagination commencing and leading to 

 the presence of a lumen. More than this, the mode of 



