THE MARINE ARACHNIDS AND THEIR ORIGIN. 339 



the primitive head from which it arose, gradually dwindles into structural 

 insignificance. 



The subdivision of the trunk into a linear series of like parts or metameres 

 was coincident with its elongation and increase in size, and there was probably 

 little difference between annelids and arthropods during the earliest stages of 

 this process. But in the former, the production of new metameres by apical 

 growth frequently persists for an indefinite post-embryonic period, and special 

 groups of metameres may separate from the parent stock by transverse fission, 

 giving rise to a succession of new individuals. 



In the primitive arthropods, the increase in the size of the body and in 

 the number of metameres proceeded very slowly. In the primitive arachnids, 

 crustaceans, and insects, the number of metameres produced was small, and 

 precisely limited; in no case was there a persistent production of new metameres, 

 either following normal, fission or otherwise. 



The production of new metameres did not take place at a uniform rate, but 

 in a spasmodic, or interrupted, manner. In the arthropods each new generation 

 of metameres consisted of a relatively small number, frequently in threes or sixes, 

 followed, after a recognizable pause, by a new generation, and so on. Thus the 

 primitive arthropod trunk consists of a small number of metameres divided into 

 groups, each group sharply distinguished by the size and the degree of special- 

 ization of its organs from the group in front or behind. The general appearance 

 is that of an annelid undergoing transverse fission, and consisting of a chain of 

 incompletely separated individuals. 



Various manifestations of this condition are seen in the nauplius, meta- 

 nauplius, and other larval stages; in the successive addition of metameres in 

 larval trilobites, and in the persistent subdivisions of the body of many other 

 arthropods into tagmata, or groups of like metameres, such as the oral, thoracic, 

 vagal, abdominal and caudal. In these cases each subdivision is produced more 

 or less clearly by a spasmodic generation of metameres, each group arising behind 

 the one previously produced. 



This phenomenon is not confined to the arthropods; it is still recognizable 

 in the subdivision of the vertebrate head, and in the successive generations of 

 nephric tubules and other organs in the post-cephalic region. One of the most 

 important events in the evolution of the ostracoderms was the addition, probably 

 during the Ordovician period, of a new generation of caudal metameres to the 

 twenty odd that constituted the sum total of their inheritance from the arachnids. 



The main difference between annelids and arthropods, besides the method 

 of apical growth, lies in the extraordinary development of the chitenous mantle or 

 exoskeleton of the latter, the presence of which no doubt exercised profound in- 

 fluence over the whole course of their evolution. Primitive arthropods, on the 

 other hand, appear to resemble the molluscan type in their restricted apical 

 growth and in the presence of the membranous folds arising from the aboral 

 region of the head, and which give rise in a suggestively persistent manner, either 



