396 THE CRANIATES AND THE ACRANIATES. 



cellate parietal eye, frontal, or olfactory sense organs, and compacted cephalic 

 neuromeres. The vast majority of them were free moving forms during their 

 early post embryonic, and adult stages. Their highly specialized appendages, 

 well developed sense organs, and neuro-muscular systems enable them to execute 

 varied movements with great precision in response to exceedingly complex sur- 

 roundings; they were the first animals to acquire an effective response to stimuli 

 of distant origin, to perceive the intangible, to pursue, and to capture. Their 

 eggs contained a large quantity of yolk, and the embryos did not leave the egg 

 till they had attained an advanced stage of development. 



In this phylum the theme was metamerism. Progress was first attained by 

 perfecting metamerism, later by its suppression or elimination. The larger 

 possibilities of this type of structure were practically exhausted in the arthropods, 

 and it had already entered another phase before the critical period arrived that 

 was to give rise to the vertebrates. During the early history of the phylum, pro- 

 gressive evolution was effected by a gradual increase in the number of metameres, 

 and by gradually increasing the perfection, or the fullness and precision with 

 which metamerism was expressed. Metamerism then began to decline, owing 

 to the local exaggeration, suppression, and fusion of organs. This process was 

 most strongly marked at the anterior, or older end of the lengthening series of 

 metameres, thus leading to the formation of an extensive and extremely complex 

 head region, to a new linear arrangement of unlike organs and functions, and to 

 the production of a higher and more unified type of organization than has been 

 attained in any other phylum of the animal kingdom. 



The formation of new metameres at the caudal end, and the specialization 

 of the older ones at the cephalic end, on the whole proceeded simultaneously, 

 so that at no stage in the evolution of the phylum did the body consist of a 

 long series of like metameres. Only a few metameres, if any, ever approached 

 a condition of ideal perfection, that is, one containing all the so-called segmental 

 organs. Serial homology in the craniate phylum is, therefore, necessarily im- 

 perfect, and the organs at one end of a series are never fully comparable with 

 those at the other. 



II. THE ACRANIATA. 



The second great group of animals with arthropod affinities constitutes 

 the acraniata. It may be called the cirriped division of the arthropod stock, 

 for the cirripeds appear to form its central figure, and because many of the more 

 striking features of the various sub-phyla are most clearly expressed in the cirri- 

 peds. We include in the acraniata, the cirripeds, tunicates, Amphioxus, echino- 

 derms, enteropneusta, chsetognotha, pterobranchia, phoronida, polyzoa, and 

 brachiopoda, all of which, with the exception of the polyzoa, are exclusively marine. 

 They are all derived from cirriped-like forms, or with them, from ostracoda, or 

 small nauplius-like arthropods that consisted of a small number of imperfectly 

 developed metameres. They form more or less independent subphyla, in no 



