28 OUTLINE OF THE ARACHNID THEORY. 



In Bothriolepis (Figs. 247 and 248), we have a similarly shaped body, with 

 similar oar-like cephalic appendages, and from the various positions in which 

 they are found in the deposits, there can be no doubt that they crawled, partly 

 buried in soft mud, with the ocular, or neural side up, but swam with the neural 

 side down, the center of gravity lying below the attachment of the arms toward 

 the bottom of the boat-shaped head. The same was probably true of Cyathaspis 

 (Fig. 244), Tremataspis (Fig. 236), Pteraspis, and probably to a less extent of 

 Cephalaspis (Fig. 232). 



Fig. 19. — Embryos of a spider in side view. 



The prevailing position among vertebrates is unquestionably with the neural 

 side uppermost, although, as we have just seen, the most primitive vertebrates 

 may move about with either side up. It is by no means true that the prevailing 

 position of the invertebrates is with the neural side down. In many annelids, 

 there appears to be no fixed position for the neural and haemal surfaces. In 

 most crawling arthropods (insects and spiders), the neural side is directed down- 

 ward, but probably in the vast majority of phyllopods, cladocera, copepods, 

 merostomes, and trilobites, and in the larvae of decapods and cirriped^, the pre- 

 vailing position, when swimming freely, is with the neural side uppermost, and 

 that is the approximate position in practically all the adult cirripeds. 



