ECHINODERMA GENERAL DESCRIPTION 



27 



are : the " respiratory trees," which occur in some Holothurians 

 as outgrowths from the cloaca ; the anal tube of Crinoids, which 

 in some Palaeozoic forms was large and with folded walls, forming 

 the so-called " ventral sac " ; the " accessory intestine " of Echinoids, 

 a kind of by-pass, permitting water to flow through without 

 interfering with the digestive process going on in the main gut. 



Lymph-glands. The amoebocytes are formed in specialised 

 glandular regions of both haemal and ambulacral systems. Of 

 the former nature are the radial and-pharyngeal vesicles of Eegular 

 Echinoids, first described by Prouho (1888) ; the greater part of the 



14 9 13 



11 

 FIG. XXIII. 



12 



14 



Echinoderm histology. 1, fundamental fibrous substance, with nuclei and an embryonic 

 cell, from Echinaster sepositus. 2, stellate embryonic mesenchyine cells of Asterias gladalis. 

 3, gelatinous connective tissue of Spatangus purpureus ( x 200). 4, elastic fibres of connective 

 tissue from Asterias glacialis. 5, fibres from stalk of a pedicellaria of same. 6, muscle-fibre 

 of same. 7, muscle-fibre from a spine attachment of Toxopneustes lividits ( x 200). 8, muscle- 

 fibre from jaw pyramid of same (x 250). 9, muscle-fibres from gut of Sphaerechinus eseulentus 

 (x 175). 10, transverse section through a muscle-bundle of Asthenosoma urens; s, sheath of 

 connective tissue from which proceed septa that limit the smaller divisions. 11, stroma 

 continued as fibrils across a suture in Spatangus purpureus, the stereom of the ossicles 

 dissolved away. 12, transversely striate muscle -fibres of Echinus acutus. 13, dorsal ligament 

 of arm of Antedon ( x 125). 14, interarticular substance of Isocrinus asteria. 1, 2, 4, 5, 6 (after 

 Cuenot). 3, 7, 8, 9 (after Hoffmann). 10 (after P. and F. Sarasin). 11, 12 (after Hamann). 

 13 (after W. B. Carpenter). 14 (after Joh. Miiller). 



Stelleroid lacunar system, just mentioned; and the " spongy organ" 

 of Crinoids in the oral ring. Connected with the ambulacral system 

 are the " Polian vesicles " found in most Echinoderms other than 

 Crinoids, and the " Tiedemann's bodies" of Asteroidea (p. 243). 



The primitive Mesenchyme cells, derived chiefly by migration 

 from the endoderm, partly from the ectoderm, have a large nucleus 

 and indistinct, often amoeboid, cytoplasm. From them are developed 

 connective and muscular fibres, amoebocytes and calcigenous cells, 

 and intercellular, gelatinous, and fibrous substances. The muscle-fibre 

 (Fig. XXIII. 6-9) derived from a single cell is smooth and straight, 



