EC HINODERM A 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 Regular 

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



14 9 13 



Fio. XXIII. 



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

 cell, from EcUinaster sepositus. 2, stellate embryonic meseuchyme cells of Asterias glacialis. 

 3, gelatinous connective tissue of Spatangus purpurevs (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 lividus(x 200). 8, muscle- 

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

 (X 175). 10, transverse section through a muscle-bundle of Asthenosomn 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 

 i >f arm of Antcdon ( 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 Echinodernis 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, 



