646 COMPARATIVE ANATOMY 



ammocoetes larva into petromyzon proves that the animal is not degener- 

 ate. The resemblance of limulus and ammocoetes is attested by many 

 anatomical details. Both, for example, have two lateral and two median 

 eyes. 



In agreement with many morphologists, Gaskell traces arthropods 

 back to an original coelenterate stock in which the central nerve cord 

 consists of a ring of nervous material which surrounds the mouth or the 

 umbrella. By the union of the two halves of the coelenterate umbrella 

 along a median line, the bilaterally symmetrical worm is formed. The 

 phylogenetic series then leads through annelids to arthropods. But, as a 

 consequence, the arthropod esophagus pierces the central nerve cord, 

 and divides it into a supraesophageal and a subesophageal ganglion. 

 Certain details of resemblance between annelids and vertebrates, such as 

 coelom, metamerism, coelomoducts, aortic arches, etc., have led to the 

 attempt to derive vertebrates directly from annelids. Since, however, 

 the central nerve cord of annelids is ventral while that of vertebrates is 

 dorsal, the annelid hypothesis of vertebrate ancestry necessitates the 

 reversal of dorsal and ventral sides of the body, and a new ventral mouth 

 to replace the old one which by reversal has been carried to the dorsal 

 side. In Gaskell's opinion "the difl&culties in the way of accepting such 

 reversal of surfaces have proved insuperable." By imagining the enclo- 

 sure of the arthropod stomach and intestine by the nerve cord and the 

 migration of the two towards the dorsal side, Gaskell seeks to avoid 

 the necessity for such reversal of surfaces. The necessity for a new 

 mouth, however, cannot be escaped, and his solution of that problem 

 will be stated later. 



It is, however, obvious that changes such as are postulated by Gaskell 

 might quite as readily take place in annelids as in arthropods. Gaskell's 

 reason for deriving vertebrates from arthropods is the higher development 

 of the arthropod brain. 



Gaskell writes: "If there is one organ more than another which 

 increases in complexity as evolution proceeds, which is the most essential 

 organ for upward progress, surely it is the central nervous system, espe- 

 cially that portion of it called the brain. This consideration points 

 directly to the origin of vertebrates from the most highly organized 

 invertebrate group — the Arthropods — for among all the groups of animals 

 living on the earth in the present day they alone possess a central nervous 

 system closely comparable with that of vertebrates. Not only has an 

 upward progress taken place in animals as a whole, but also in the tissues 

 themselves a similar evolution is apparent, and the evidence shows that 

 the vertebrate tissues resemble more closely those of the arthropod than 

 of any other invertebrate group. 



