CHAPTER XIV. 



THE OLD MOUTH AND THE NEW; LOCOMOTOR AND RESPIRA- 

 TORY APPENDAGES. 



The closing of the invertebrate mouth, the formation of a new one, and the 

 evolution of segmental appendages into leg-jaws, gill-sacs, and locomotor append- 

 ages are complex independent processes, but they are so interwoven with one 

 another in the early history of the vertebrates that they may be appropriately 

 treated together. 



The salient features of the mouth and appendages in arthropods and verte- 

 brates may be contrasted as follows: 



a. In arthropods, the mouth lies on the neural surface, and the foregut passes 

 through the brain floor between the two nerve cords, just behind the forebrain. 

 (Fig. 43.) In vertebrates the mouth lies on the hsemal surface of the head, and 

 the foregut, without passing through the brain floor, leads directly into the mid- 

 gut. (Fig. 44.) 



b. In arthropods there are many pairs of appendages, the most conspicuous 

 ones being arranged in rigid metameric order either on the sides or on the neural 

 surface of the first thirteen to eighteen metameres. They may be absent in some 

 metameres, while in others they assume a great variety of forms suitable for loco- 

 motion, sense organs, jaws, gills, etc. (Fig. 3, A.C.) In primitive vertebrates, 

 metamerically arranged appendages like those of arthropods, appear to be 

 absent. The paired locomotor appendages, when present (pectoral and pelvic 

 fins) are not segmentally arranged; they are merely local expansions of longi- 

 tudinal folds, and they always lie posterior to the (i6 ) metameres that constitute 

 the head. 



c. In arthropods the jaws are formed from several pairs of modified legs that 

 belong to the metameres lying just behind the forebrain. The basal joints of the 

 leg-jaws act as crushing mandibles, or as supplementary jaws. In chewing, 

 tasting, or preparing food, they work crosswise, to and from the median neural 

 line. In true vertebrates the jaws, in the adult stages, consist of two unpaired 

 arches, or an upper and a lower jaw. They lie on the haemal surface instead of 

 the neural, and in chewing move forward and backward instead of crosswise. 



d. In the arthropods, respiration is usually accomplished by means of 

 specially modified appendages that either project outward above the surface of the 

 body, or inward, forming ectodermic pouches, with vascular, lamellate walls. 

 In fishlike vertebrates the gills may, for a brief early period, consist of external 

 appendages of ectodermic origin, but in their later stages they consist of 



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