THE EVIDENCE OF THE SKELETON 147 
In the invertebrate kingdom true cartilage occurs but scantily. There is 
a cartilaginous covering of the brain of cephalopods. 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; 
and 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 Ammoceetes is 
extraordinarily great. 
Here, in Limulus, just as in Ammoceetes, an internal cartilaginous skeleton 
is found, composed of two distinct parts: (1) prosomatic, (2) mesosomatic. As 
in Ammoceetes, the latter consists of simple branchial bars, segmentally arranged, 
which are connected together on each side by a longitudinal ligament contain- 
ing cartilage—the entapophysial ligament. This cartilage is identical in 
structure and in chemical composition with the soft cartilage of Ammoccetes, 
and, as in the latter case, arises in a markedly mucoid connective tissue. 
The former,-as in Ammoccetes, 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-formation 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 
obseryations 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 Palwostraca, and is 
of so strong a character that, taken alone, it may almost be considered as proof 
of such origin. 
