Anatomy of Gorgonid^. 



[09 



brata, is derived from the mesoblastic plates and hence is 

 mesoblastic in origin. 



We thus find that the calcareous endoskeleton of all the 

 more prominent groups of animals possessing them, are essen- 

 tially homologous in origin,' being all derived from the 

 mesoblast. 



This homology is all the more significant when we con- 

 sider the widel}'- different structures entering into the skeletons 

 of the groups enumerated. 



To return to the question of organic matter in intimate 

 union with the earthy salts in the spicules, it is well to remem- 

 ber that while bone contains much organic matter it is derived 

 from many cells; spicules, on the contrary, seem each to be 

 derived from a single cell within which the deposition com- 

 mences. Therefore we can not consider the composition of 

 the former as a clue to that of the latter as has sometimes 

 been done. 



Basal Substance. This is what is called '■'■substance 

 fondamentalc " by Pouchet and Myevre and seems to me to 

 be more correctly thus called than by any of the names, such 

 as coenenchyma, sarcosome, coenosome, coenosarc, etc., which 

 have been applied to it by other writers.'' As used in this 

 paper, the name signifies the substance which forms the 

 matrix, as it were, in which the various histological ele- 

 ments are embedded. It is in fact the mesodermal, structure- 

 less, hyaline substance constituting a large proportion of the 

 entire mass. 



It is the substance in which the spicules are embedded and 

 through which the water-vascular canals penetrate in their 

 various ramifications. It forms the outer ' layer of the calicle 

 lining and sends forth the apparently structureless mesoder- 



I An apparent exception to this rule is found in the calcareous skeleton of 

 Millepora and Stylasteridie which is said to be epiblastic. Idem., p. 144. 



3 " Nous appelons de ce nom . . la substance particuli&re an milieu de 

 laquelie sont places on sur laquelle s'appuient tous les 616ments anatomiques de 

 I'animal. Elle constitue essentiellement le coenenchyme de M. Edwards." 

 Pouch ?t et M\'fevre. Contribution a 1' Anatomic des Alcyonaires. p. 288. 



3 Outer, counting from the center of the calicle. 



