268 THE OLD MOUTH AND THE NEW. 



The most anterior of the lateral gut pouches in the vertebrates corre- 

 spond with those of the locomotor appendages of the arachnids. They probably 

 atrophied in the immediate ancestors of the vertebrates, neither communicating 

 with the exterior, nor leaving any definite organs behind. ' The paired haemal 

 outgrowth of the thoracic gut probably persists as the thyroid gland, with which 

 it agrees in position and direction of growth. (Figs. 43, 44, 308.) 



In the arachnids the free ends of the lateral and of the haemal enteric pouches 

 of the thorax are usually branched, or lobular, or racemous, the subdivisions con- 

 sisting of a single layer of cylindrical secreting cells that suggest, in their general 

 appearance, the condition presented by the thyroid in vertebrates, and by those 

 organs resembling the thyroids that arise from the walls of the visceral clefts. 



The thymus probably represents a modification of the several pairs of thoracic 

 coxal glands that occur in the arachnids. 



The postbranchial outgrowths of the vertebrate mesenteron, i.e., the lungs, 

 liver, and pancreas may be regarded as local specializations of enteric diver- 

 ticula comparable with the endodermic portions of the visceral clefts. (Figs. 

 181, 182.) 



VII. The Locomotor Appendages. 



■A. The Cephalic Appendages. — In the higher arthropods, locomotionis effected 

 by several pairs of jointed appendages arranged in strictly segmental order, and 

 usually located anterior to the respiratory region. (Figs. 3, 4, 5.) In true verte- 

 brates the locomotor appendages are always situated behind the gills; there are 

 never more than two pairs ; and they have no fixed relation to the metameres. (Fig. 

 4, B and C.) 



The meaning of this sharp contrast will appear after a more careful examina- 

 tion of their structure and serial location in the two groups, and in the intermediate 

 one formed by the ostracoderms. 



In free swimming phyllopods and arachnids, there may be one pair of large 

 oar-like appendages that arise from the anterior end of the thorax, as for example 

 the antennae of many cladocera and entomostraca, or the elongated chelicerae of 

 Pterygotus. Or such a pair may arise from the posterior part of the thorax, as in 

 Eurypterus. (Fig. 5.) In all these cases the locomotor appendages lie in front of 

 the respiratory region. 



In typical vertebrates the paired locomotor appendages, if present, consist 

 primarily of two lateral folds that extend from the postbranchial to the precaudal 

 region. The pectoral and pelvic fins are local expansions of these folds. The 

 paired fins are not definite segmental structures, for they consist of muscle buds and 

 cartilages derived from a large and varying number of metameres. 



As there are approximately sixteen or more metameres in the vertebrate head, 



1 Possibly they may be represented by the "lateral thyroids" and by the diverticula leading into the 

 cement glands of amphibia. 



