394 THE CRANIATES AND THE ACRANIATES. 



The clue was apparently leading in the wrong direction, into the darkness 

 rather than into the light, and the problem was by no means simplified when it 

 appeared, more and more clearly, that Balanoglossus resembled Amphioxus and 

 the tunicates in certain important particulars, especially in the structure and 

 development of the ccelom and gill clefts, while its larva resembled that of the 

 echinoderms. 



Again the problem was still further complicated by the discovery of Cephalo- 

 discus and Rhabdopleura, at first supposed to be related to the polyzoa, and con- 

 sequently suspiciously close to the brachiopods, but later very generally recog- 

 nized as also related to Balanoglossus, and hence in some way involved with the 

 tunicates and amphioxus, which outwardly they did not in the least resemble. 

 An amphioxus-balanoglossus-like animal with six pairs of legs, such as those 

 of Cephalodiscus, was perforce accepted without a grimace, although it was not 

 very readily assimilated. 



The trail was leading well down toward the roots of the animal kingdom, 

 but certainly not toward anything like a worm-like ancestral form composed of 

 many well defined, similar metameres. The evidence that was accumulating, 

 while in some cases more concrete than that produced in the earlier history of the 

 problem, became less convincing. It led in too many directions, and was forcing 

 morphologists at large to accept conclusions against which their better judgment 

 rebelled, but from which there was apparently no escape. While many preferred 

 to doubt the evidence rather than accept the conclusions, or even began to lose 

 faith in the efficacy of comparative embryology as a means of solving large problems 

 in phylogeny, others, with commendable loyalty, adhered to the particular faith 

 in which they had been educated, and advocated it with sufficient ardor, at least 

 until the next hypothesis appeared. But as the number of attractive theories 

 increased, the older morphologists apparently concluded that it was wiser to 

 accept neither one nor the other, and to beware of them all. It was perhaps 

 realized that one might live very happily wedded to one view, if it was not for 

 the others; for it was increasingly evident that embracing any one theory created 

 more difficulties than were overcome, since each rejected one was then sure to 

 look more formidable than ever. 



However, our new hypothesis is not open to these objections, for in accepting 

 the arachnid theory we shall have the privilege of adopting into our household, 

 as her children, many of the other attractive theories that have from time to time 

 won our affection. The arachnid theory opens the way to a reconciliation of the 

 conflicting views above indicated. It offers a solution that is logical, consistent 

 with the facts, so far as we know them, and in harmony with the basic principles 

 of morphology. 



In brief, we recognize two great groups of animals that have independently 

 acquired some of those characters commonly associated with the chordata. 

 Both groups are descended exclusively from primitive arthropod stock, that is, from 

 small bodied animals of a small number of ill defined metameres (resembling a 



