8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. J2. 



THE ECHINODERM SKELETON 



MacBride and others have remarked that the presence of calci- 

 fied skeletal tissue in the mesoderm of the body wall is a character 

 found in the echinoderms and vertebrates only among coelomate 

 animals. It does not seem to me that this can be considered as any 

 indication of affinity between these groups. 



Many vertebrates have uncalcified mesodermal skeletons, and 

 others have only partially calcified skeletons. The uncalcified skeleton 

 of the notochord resembles in structure the parenchyma of the solid 

 tentacles of certain coelenterates, and is quite different from anything 

 found in the echinoderms. Calcareous deposits of greater or lesser 

 extent occur in the mesoderm of barnacles, brachiopods, rotifers and 

 cestodes, as well as in the mesogloea of sponges, and the calcareous 

 skeletal structures of some coelenterates are mesodermal in origin. 



In certain of the early cystideans the skeleton appears to have 

 been wholly or chiefly chitinous, and their surface exactly resembles 

 that of the phyllopod and other crustaceans preserved in the same • 

 rocks. It is not improbable, therefore, that the calcareous exoskeleton 

 of the echinoderms of the present seas developed from a chitinous 

 body covering through an exoskeleton composed of chitin with an 

 increasingly greater amount of inorganic matter such as we see today 

 in most of the larger crustaceans. 



In the developing crinoid the ectoderm of the surface of the body 

 more or less completely disintegrates and its component cells largely 

 pass inward and intermingle with the cells of the underlying meso- 

 derm, so that in the crinoid the outermost layer of the body is almost 

 as much mesodermal as it is ectodermal. This, being the case, no 

 matter what its phylogenetic relationships and tendencies are, the 

 formation of an ectodermal skeleton has now become impossible as 

 there is no continuous ectoderm from which to form it. A cal- 

 careous mesodermal skeleton appears, the first rudiments of which 

 are formed in the deeper layers but soon move to a more superficial 

 position enclosing the body in a calcareous investment formed of large 

 and definite plates. Just before the appearance of the arms there are, 

 in addition to the columnals, 13-15 thin cribriform films lying just 

 below the surface and fitted edge to edge, including 3-5 infrabasals, 

 5 basals alternating with them, and 5 orals superposed upon the 

 latter. 



Now although this skeleton is mesodermal and calcareous, the re- 

 lations between it and the enclosed body of the animal are entirely 

 different from the relations between the vertebrate skeleton and the 



