89 



organ of respiration, in which a constant current is kept up 

 by means of ciUa. In the third part of this chapter we shall 

 speak of the occurrence of hemoglobin in Ophiactis virens. 



In the Holothurians we have very definite organs of respi- 

 ration, the so called water-lungs ') or respiratory trees in which 

 an alternating in- and outflow of water through the cloaca can 

 be observed. They have a more or less tree-like appearance, 

 the stem of the tree being represented by the two main tubes 

 opening into the cloaca. The finer branches of these gills end 

 in rounded, thin-walled swellings, termed ampullae. The rhythm 

 of the process of in- and outflow appears to comform to van 

 't Hoff's rule 18^). The inflow seems to take place inter- 

 mittently, the outflow however in one strong current (Peach. 

 Ann. Mag. Nat. Hist. Vol. XIII. 1881. p. 418; Ayres 

 (Thyone). Proc. Boston Soc. Nat. Hist. Vol. IV. 1851 — 1854; 

 Sluiter. Natuurk. Tydschr. v. Nederl. -Indie. Bd. XL. Batavia. 

 1880). They are not the chief organs of excretion, asHerouard 

 supposed, but phagocytes loaded with granules (excretion 

 products) or artificially injected substances, leave the body 

 through their wall. 



Since however as we have pointed out in the preceding chapter, 

 these „nephrocytes" leave the body by almost any route provided 

 that oxygen is present, this is not a specific function of the 

 respiratory trees. 



One observation certainly is an argument for their respira- 

 tory function. The animals which I used were kept in a very 

 shallow aquarium with a standing overflow. Now, occasionally 

 several specimens could be seen to move their cloaca upward 

 above the surface of the water and to open it -to the air. 

 This attitude is frequently kept up for a considerable length 

 of time and has apparently some importance for respiration. 



The same behavior was observed carefully by Winter- 

 stein 133), who found that it occurred especially in water 

 which had been polluted „durch den Abgang des Exkremente". 



It is not a pure air-breathing act. The lungs are kept full of 

 water and this water is even changed every now and then — 

 the cloaca again being moved under the surface — ^.The gaseous 

 exchange between the water and the air is greatly facilitated 

 by this procedure, however, as analogous cases show (compare 

 e. g. the larvae of Aeschnidae and the buccal respiration of 

 fishes, their „Notatmung"). In case of lack of oxygen the ani- 

 mals first contract, then they relax and faint, finally they lose 

 all tonus and „Reaktionsfahigkeit". 



One thing which certainly proves that they are not the only 

 organ concerned with respiration, is the fact that they are so 



^) The water-lungs have also frequently been supposed to be lymph glands 

 (e.g. by Herouard) and organs pf excretion (see the chapter on this topic). 



