10 



OUTLINE OF THE ARACHNID THEORY. 



were used as supplementary swimming, or respiratory appendages, or for crawling 

 or walking, and the circulatory organs appeared in the haemal region. (Fig. 308.) 

 The internal organs, such as the stomach, digestive glands, gut pouches, 

 organs of excretion and generation, establish their relations to the rest of the body, 

 if at all, through the circulation. They are less dependent on location, or on 



ol.o. 



FIG. 4. Diagrams showing location of principal organs in Bothriolepis and a primitive vertebrate. A, B, Haemal; 

 view; C, neural view. .-1, Bothriolepis; B, C, primitive vertebrate. 



specialization in form, for effective action, hence they are eventually crowded into 

 the more posterior metameres, or they atrophy and new ones arise farther back 

 to take their places. (Figs. 307, 308.) 



In the typical arachnids, a definite linear arrangement of unlike functions, in 

 accordance with the above principles, is established at an early period. The 

 order is essentially the same as that in the vertebrate head, and is as follows: 

 olfactory, coordinating, visual, swallowing, gustatory, auditory, locomotor, equili- 

 bratory, cardiac, and respiratory. (Figs. 5, 57 and 114.) 



In the posterior cephalic regions, the digestive, excretory, and genital organs 

 are closely associated with, or overlap, the branchial and cardiac organs, this 

 arrangement forming a conspicuous feature in the arachnids. It appears to be 

 retained, to a large extent, in Bothriolepis and other ostracoderms. (Fig. 5.) In 

 the vertebrates, this arrangement is further modified by the atrophy of the pre- 

 branchial locomotor appendages, by the formation of new ones behind the gills, 



