HOLOTHUROIDEA. 259 



which opens into the cloaca and may represent them. The number of 

 respiratory trees is never more than two, the apparent exceptions to this 

 rule being caused by some of the branches acquiring a great distinctness. 

 The function of these organs is probably respiratory, the cloaca apparently 

 having the power of sucking up water and of driving it into them, and then 

 of expelling it. Rhythmical inspiratory and expiratory movements of 

 the cloaca and to a certain extent of the body appear to effect this. They 

 may also be partly excretory, for the expelled water besides carrying 

 faeces also contain various kinds of cell debris including cells with brown 

 granulations, which probably originate on the walls of the " trees." 



The euvierian organs (Fig. 181, 14) are tubular organs which open into 

 the terminal parts of the respiratory trees. The number varies in different 

 species, but as many as 100 have been counted in one individual. They 

 are usually unbranched, but they may be branched or even racemose. 

 They are found mainly in the Aspidochirotae, especially in the genera 

 Holothuria and Mulleria, but they appear to be entirely absent in Labi- 

 dodemas, Pseudostichopus, Paelopatides, and Stichopus (St. paradoxus 

 excepted). They are unknown in Synaptidae ana Elasipodidae and are 

 only exceptionally found in other families (e.g. Molpadia chilensis, 

 Cucumaria frondosa and nigricans). They are probably to be regarded 

 as modified branches of the respiratory trees. The unbranched tubes 

 are lined with an epithelium, outside which is a layer of connective tissue. 

 This is followed by an internal circular and an external longitudinal 

 muscular layer, the internal circular layer consisting of a closely wound 

 spiral fibre. Then comes an outer connective tissue layer and finally the 

 peritoneal epithelium, which in the case of the glandular euvierian organs 

 appears to be peculiarly modified and to secrete a sticky substance. In 

 some Holothurians (the so-called cotton-spinners) the euvierian organs 

 can be ejected from the cloaca when the animal is irritated and used as 

 organs of defence. This phenomenon has been studied in Holothuria 

 nigra (forskali),* and in H. poli,-\ and in other forms. % When the skin 

 is irritated a small number of these organs make their .way, blind end 

 forwards, through a rent which is formed in the dorsal wall of the cloaca. 

 On emerging from the anus they rapidly undergo elongation to twenty or 

 thirty times their original length, darting about in all directions and be- 

 coming attenuated in the process. They stick by their viscid surface to 

 everything they touch (except the surface of the animal itself). If the 

 Holothurian now moves away they become detached from its body by 

 rupture. The cause of the active elongation and movement of the tubes 

 is not clearly known. It is not apparently due to injection of fluid from 

 the respiratory trees because it can take place if the tubes are detached 

 from the animal. It would appear to be caused by some process taking 

 place in the wall of the tube itself. When first ejected each tube is 

 thicker at the free end than at the base. The elongation appears to take 

 place at the expense of this " head " which diminishes in length during the 

 process. The elongation is said to begin while the organs are still in the 

 body-cavity and can be brought about by direct irritation of the tubes 

 themselves as well as of the skin. The tubes after elongation cannot be 

 shortened and must therefore be cast off, new tubes being presumably 



* Minchin, Ann. and Mag. Nat. Hist. (6), 10, 1892, p. 273. 



f Barthels, Verh. Nat. Ver. Bonn, 53, 1896, p. 76. 



J Semper, op. cit. Peach, Ann. and Mag. Nat. Hist., 15, 1845, p. 171. 



