330 THE ORIGIN OF VERTEBRATES 
star-like cells, however, are not arranged concentrically around these 
spaces, aS in true Haversian canals. 
This structure is therefore a calcareous infiltration of a tissue 
with cells in it. Where is there anything like it? 
As soon as I saw Rohon’s picture (Fig. 130), I was astounded 
at its startling resemblance to the structure of muco-cartilage as is 
seen in Fig. 131, taken from Ammoccetes. If such muco-cartilage 
were infiltrated with lime salts, then the muco-cartilaginous skeleton 
of Ammoccetes would be preserved in the fossil condition, and be 
comparable with that of Cephalaspis, etc. 
Fic. 180.—Srcrion or a Hxrap- Fic. 181.—Sscrion or Muco- 
PLatE oF A  CEPHALASPID. CarTibaGe FROM DorsaL 
(From Roxon.) HeEaAv-PLATE OF AMMOC@TES. 
The whole structure is clearly remarkably like Rohon’s picture of 
a section of the head-plate of a Cephalaspid (Fig. 130). In the latter 
case the matrix contains calcium salts, in the former it is composed 
of the peculiar homogeneous mucoid tissue which stains so charac- 
teristically with thionin. With respect to this calcification, it is 
instructive to recall the calcification in the interior of the branchial 
cartilages of Limulus, as described in Chapter IIL, for this example 
shows how easy it is to obtain a calcification in this chondro-mucoid 
material, With respect to the medullary spaces and smaller spaces in 
this tissue, as described by Rohon, I would venture to suggest that they 
need not all necessarily indicate blood-vessels, for similar spaces would 
appear in the head-shield of Ammoccetes if its muco-cartilage alone 
