40 OUTLINE OF THE ARACHNID THEORY. 



The Cornua of the cephalothoracic shield are retained as the opercular 



fold, which extends over the posterior appendages, as suggested for Cepha- 

 laspis, to form a respiratory, or atrial chamber. The projecting margins of the 

 abdominal segments or pleurites are retained as the lateral fold, from which the 

 paired, post-branchial appendages arise. (Figs. 29-34.) 



The Oral Arches and the Haemostoma. — Both the formation of the 

 haemostoma, or new mouth, and the transfer of the basal joints of the anterior 

 thoracic appendages to the hsemal side of the head to form the oral arches, are 

 the inevitable results of the processes that have been steadily going on during the 

 phylogeny of the arachnid cephalothorax. These processes are: the increased 

 size of the yolk sphere; the increased size of the forebrain neuromeres; and the 

 progressive degeneration of the cephalic mesoderm. The way in which these 

 changes affect the location of the jaws and the shape of the head, during the early 

 stages of development, is shown in Figs. 31-34. It will be seen that as the fore- 

 brain increases in volume and in precocity, the apex of the head is elevated and 

 thrust forward off the surface of the egg. As the haemal ends of these anterior 

 metameres are greatly reduced in volume, or absent, the other head structures, 

 which were originally neural or lateral in position, such as the anterior meso^ 

 blastic somites and the appendages, are drawn toward the haemal side of the head. 

 Here they converge around the ingrowing, haemal surface, or cephalic na^•el 

 (dorsal organ) that represents the beginning of the buccal infolding, the basal 

 joints of the appendages forming the beginning of several pairs of oral arches, i.e., 

 premaxillae, maxillae, mandibles, and hyoids. 



The more posterior arches are not subject to these conditions; hence they 

 tend to remain in their original position on the neural or lateral surface. But 

 in the later stages of the higher vertebrates, even they may be transferred to the 

 haemal surface. (Figs. 307, 308.) 



The mouth parts of our embryo are now in the ostracoderm and cyclostome 

 stage, one that is seen temporarily in all higher vertebrate embryos (see develop- 

 ment of the jaws in the frog) (p. 257), and permanently in the adult cyclostomes 

 and ostracoderms. (Figs. 159-174.) The true vertebrate condition is attained 

 by the union of two pairs of arches to form a single, fixed, upper jaw, and the 

 union of a third pair to form a movable, unpaired, under jaw, or mandible. 



The older stages of our arachnid-vertebrate embryo, Fig. 34, are character- 

 ized by an increase of the cranial flexure, bringing the heart close under the 

 anterior end of the brain, and producing that forward dislocation of the hypo- 

 branchial muscles, so characteristic of vertebrates ; by the opening of the gut pouches 

 into the lung-books; by the appearance of true vertebrate appendages as post- 

 cephahc outgrowths of the marginal fold; by the increasing size of the cartilage 

 cranium and gill bars; by the substitution of a subdermal skeleton for an epidermal 

 one, and by the conversion of the arthropod lematochord into the notochord. 



