THE EVIDENCE OF THE SKELETON 1 47 



In the invertebrate kingdom true cartilage occurs but scantily. There is 

 a cartilaginous covering of the brain of cepkalopods. It is never found in crabs, 

 lobsters, bees, wasps, centipedes, butterflies, flies, or any of the great group of 

 Arthropoda, except, to a slight extent, in some members of the scorpion group, 

 aud more fully in one single animal, the King-crab or Limulus : a fact significant 

 of itself, but still more so when the nature of the cartilage and its position in 

 the animal is taken into consideration, for the identity both in structure and 

 position of this internal cartilaginous skeleton with that of Anmiocoetes is 

 extraordinarily g-reat. 



Here, in Limulus. just as in Aminoccetes, an internal cartilaginous skeleton 

 is found, composed of two distinct parts : (1) prosomatic, (2) mesosomatic. As 

 in Ammocoetes, the latter consists of simple branchial bars, segmentally arranged, 

 which are connected together on each side by a longitudinal lig'ament contain- 

 ing cartilage — the entapophysial ligament. This cartilage is identical in 

 structure and in chemical composition with the soft cartilage of Ammocoetes, 

 and. as in the latter case, arises in a markedly mucoid connective tissue. 

 The former, as in Ammocoetes, consists of a non-segmental skeleton, the 

 plastron, composed of a white fibrous connective tissue matrix, an essentially 

 gelatin-containing tissue, in which are found nests of cartilage cells of the 

 hard cartilage variety. 



This remarkable discovery of the branchial cartilaginous bars of Limulus, 

 together with that of the internal prosomatic plastron, causes the original diffi- 

 culty of deriving an animal such as the vertebrate from an animal resembling" 

 an arthropod to vanish into thin air, for it shows that in the past ages when the 

 vertebrates first appeared on the earth, the dominant arthropod race at that time, 

 the members of which resembled Limulus, had solved the question ; for, in addition 

 to their external chitinous covering, they had manufactured an internal cartila- 

 ginous skeleton. Not only so, but that skeleton had arrived, both in structure 

 and position, exactly at the stage at which the vertebrate skeleton starts. 



What the precise steps are by which chitin-f ormation gives place to chondrin- 

 formation are not yet fully known, but Schmiedeberg has shown that a substance, 

 glycosamine, is derivable from both these skeletal tissues, and he concludes his 

 observations in the following words: ''Thus, by means of glycosamine, the 

 bridge is formed which connects together the chitin of the lower animals with 

 the cartilage of the more highly organized creations." 



The evidence of the origin of the cartilaginous skeleton of the vertebrate 

 points directly to the origin of the vertebrate from the Palfeostraca, and is 

 of so' strong a character that, taken alone, it may almost be considered as proof 

 of such origin. 



