CHAPTER XVII. 

 THE ENDOCRANIUM, BRANCHIAL AND NEURAL CARTILAGES. 



I. The Endoskeleton of Arachnids. 



Many arthropods are provided with an elaborate system of ectodermic in- 

 foldings, lined with chiten, that serve for the attachment of muscles and in some 

 cases as a supporting framework for the anterior part of the nervous system. 

 They may be segmentally arranged, and in some cases appear to be the modified 

 remains of duct-like infoldings that originally served some other purpose than for 

 the attachment of muscles. There are no indications that these structures are 

 represented in vertebrates, and we merely refer to them here in order to emphasize 

 the distinction between them and the true endoskeleton we shall describe in the 

 following pages. 



The endoskeleton of arachnids (Limulus) consists of four distinct parts, their 

 relation to the corresponding parts in the vertebrate skeleton being sufficiently 

 indicated by their names. They are: a. the neural arches; b. the branchial car- 

 tilages; c. the endocranium; d. the notochord. (Fig. 209.) 



■' br.ct. 



'~~^-en.t. 



Fig. 209. — Diagram of the endoskeleton of a marine arachnid (based on Limulus) showing the locations of the 

 lemmatochord, endocranium, neural arches (dotted), branchial cartilages (black), and the chitenous entapophyses 

 (shaded) . 



With the exception of the notochord, these structures are primarily meso- 

 dermic in origin; they serve for the attachment of muscles, and with the possible 

 exception of the branchial cartilages, their origin and development was determined 

 by the development of the muscles now associated with them. They have the 

 same general form, location, consistency, and chemical reaction that the corre- 

 sponding cartilages have in vertebrates. 



The arthropod notochord is a modification of the middle cord and is primar- 

 ily ectodermic in origin. The median nerve derived from it degenerates and the 

 remnants become invested with a thick envelop of neuroglia-like tissue that may 



306 



