322 ENDOCRANIUM, BRANCHIAL AND NEURAL CARTILAGES. 



union of their anterior ends; d. the closing up of the pituitary foramen; and e. the 

 separation of a part of the haemal arcade from the basilar plate to form the pterygo- 

 quadrate arcade. 



II. Branchial Skeleton. 



1. a. In Limulus the first six pairs of appendages, i.e., those belonging to the 

 same metameres as the endocranium, never develop appendicular cartilages. In 

 the seven following appendages, the bars are present; they are theiefoie post- 

 cranial in origin, although the first pair (chilarial) is firmly attached to the pos- 

 terior end of the basilar plate, b. The haemal ends of the branchial bars, except 

 the first pair, are united by a longitudinal band of cartilage, c. The gill bars and 

 the longitudinal band are composed of capsuliginous mucoid cartilage, quite 

 different chemically and histologically from the cartilage of the endocranium; 

 and d. The three pairs of haemal processes in Limulus have a superficial resem- 

 blance to the gill bars in position and direction, but they are of a different nature 

 in structure and origin, and belong primarily to the endocranium. 



2. Similar conditions prevail in vertebrates: a. The true branchial bars 

 are independent, post-cranial structures, the anterior pairs being joined second- 

 arily with the cranium, b. The segmentally arranged gill bars may be united by 

 continuous longitudinal bands, c. The gill bars and bands are composed of 

 muco-cartilage differing chemically and histologically from the fibro-cartilage of 

 the endocranium. d. The four or five preauditory metameres (excluding the 

 forebrain region) never give rise either to typical or fully developed gills, or to 

 gill bars. e. The proximal portion of the palato-pterygo-quadrate-hyoman- 

 dibular arcade does not represent modified gill arches. It belonged originally 

 to the primordial cranium and represents in part the haemal plate arcade of the 

 arachnid endocranium. (Fig. 220.) 



III. Neural Arches. 



\ 



The neural arches of Limulus represent the initial stages in the formation of 

 a vertebral column. Their contour is due to the direction and intensity of the 

 muscular strain acting on them, and they agree in general form and in the direc- 

 tion of their processes, with the neural arches of vertebrates. 



