32 OUTLINE OF THE ARACHNID THEORY. 



surface to form a true atrial chamber that encloses the gills and cloaca. (Fig. 5.) 

 The jointed, oar-like appendages, which belong to one of the posterior meso- 

 cephalic segments, are attached to the angle of the cornua, that are here very small 

 compared with those of Cephalaspis. 



Bothriolepis retains the hinge-like joint in the vagus region, which is such 

 a prominent feature in trilobites, merostomes, and other arachnids. The same 

 joint is a conspicuous feature in Dinichthyes, Coccosteus, etc., a group of primitive, 

 fish-like animals that probably unite the typical ostracoderms with the true 

 vertebrates. (Fig. 250.) 



In Tremataspis (Figs. 236 and 237), there are probably several pairs of 

 small cephalic appendages, comparable with external gills, that protruded from 

 the openings on the oral surface; the larger, oar-like pair, at the beginning of the 

 series, being especially noteworthy. The exhalent branchial currents and the 

 excretory products, no doubt pass out of the posterior end of the atrial chamber, 

 as in Bothriolepis. 



The assumed changes above described affect, in the main^ the external form 

 of the animal. The internal structure might remain essentially as' it now is in 

 arachnids, and, except for certain organs, it would harmonize with the structural 

 plan in vertebrates. For example, it would be necessary, in order to complete the 

 transformation of an arachnid into a vertebrate, to close the old mouth and to 

 connect the new one and the gill pouches with the enteron. The factors involved 

 in these changes are described elsewhere. For the present, it is enough to recog- 

 nize the fact that these events have taken place, in some way and at some time, 

 whatever the method or cause may have been. 



On the other hand it will be observed that many internal organs, that we are 

 accustomed to consider as characteristic of vertebrates, are already present in 

 the arachnids, in their proper position and relations, and merely have to be en- 

 larged or improved, or even left as they are, to agree with those in vertebrates or 

 ostracoderms. 



For example, there is already present in Limulus, in addition to the brain, 

 sense organs, and other structures that have been considered, a head kidney, 

 cox. 0., heart, h, aortic arches, a. 0., and cardinal sinuses, card. s. foreshadowing 

 those in vertebrates. (Fig. 2.) There are infolded gill sacs and gut pouches in 

 arachnids, that are precursors of the gill clefts, thyroids, and other enteric di\er- 

 ticula in vertebrates. (Figs. 179-182.) There is in Limulus and other arachnids 

 a large cartilagenousendocranium and gill bars, so similar in form, location, and 

 histological structure to those of vertebrates, that they might readily pass for 

 those of some primitive, unknown member of that class. (Figs. 210-220.) There 

 is, in Limulus, an internal, dermal skeleton, made of chiten, it is true, but never- 

 theless consisting of a network of trabeculae, cancellse. Haversian canals, lacunae, 

 and canahculae, so much like those of certain ostracoderms (Pteraspis) that it is 

 doubtful whether fossilized fragments of one skeleton could be distinguished from 

 those of the other, if their real origin was unknown. (Figs. 196-207.) And 



