28 



ECHINODERMA GENERAL DESCRIPTION 



clearly defined at the ends, with a lateral nucleus. A few striated 

 muscle-fibres are known in Echinoidea (Geddes and Beddard, 1881 ; 

 Hamann, 1 887). A semi-muscular, hyaline tissue of wavy, nucleated 

 fibrils is peculiar to Crinoidea, and is called "ligament tissue." 

 There are also muscles of endothelial origin. Connective tissue 

 fibrils are nucleate and vary in length and shape ; there are also 

 rounded or stellate cells (Fig. XXIII. 4, 2). Intercellular substance, 

 secreted by mesenchyme cells, often attains great thickness in 

 the integument ; it may remain a soft jelly, or become tough as 

 indiarubber, or may split up into interlacing fibrils ; it usually 

 contains amoebocytes and ordinary connective tissue cells ; it forms 

 also interarticular substance (Fig. XXIII. 14), elastic ligament, 



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FIG. XXIV. 



Stereom formation. 1, from the hinder portion of a Plvteus of Echinus miliaris. s, one of 

 the large supporting rods of the Pluteus ; c, a three-pronged spicule surrounded by a group of 

 calcigenous cells, which derive their lime through a meshwork of pseudopodia and cells (a), 

 from the rods of the Pluteus and from their broken ends, which are seen just below f . 2, 3, 

 earliest stages of a spicule of Echinocyamus, surrounded by calcigenous cells. 4, infra basal of 

 an Antedon larva forty-eight hours old (x 230). 5, regular stereom from the outer part of the 

 cup of JIolopus (x 56). 6, portion of horizontal section of Holopus cup, showing relation of 

 irregular (i) and regular (r) stereom (x 15). 1-3 (after Theel). 4 (after Seeliger). 5, 6 (after 

 P. H. Carpenter). 



and the walls of internal organs. Parallel structures are found in 

 the cartilage of Vertebrata. 



The formation of a calcareous skeleton by the mesoderm was 

 as pronounced in the oldest known Echinoderms as it is to-day, 

 indeed, more so. To the prickly skin, so commonly a result of 

 this, is due the name of the phylum (ex" 70 ^ a hedgehog; Sep/za, skin). 

 Amoeboid cells in the mesenchyme have the power of fusing by 

 pseudopodia into plasmodia or into reticular tissue (Fig. XXIV.). 

 Where the pseudopodia meet and fuse, the protoplasm secretes 

 a small calcareous spicule (intracellular, Theel ; extracellular, 

 Semon), which gradually increases in size along the lines of the 

 pseudopodia. Such spicules meet and fuse by their processes, 

 thus building up a hard tissue ("stereom"), with a structure that 

 in section appears reticular, but really is more like a beam-and- 

 rafter-work. As, in the growing Echinoderm, the protoplasmic 



