84 SURVEY OF INVERTEBRATES 



lices within the hydatid cyst., These morphological ob- 

 servations gave probability to the assumption that anaer- 

 obic processes occur of which the endproduct is fat. The 

 hydatid fluid itself contains succinic, valeric, propionic, 

 acetic and lactic acids (Flossner, 1924, 1925). Lactic acid 

 probably occurs also in the cysts of Cysticercus tenuicol- 

 lis according to Schopfer (1932). All these acids are 

 common endproducts of anaerobic processes in inverte- 

 brates. One cannot, however, be quite specific as to their 

 origin here. The properties of the internal fluid of the 

 vesicular stages of cestodes liken it to a transudate in 

 equilibrium with the blood plasma of the host ( Schopfer, 

 1932; Wardle, 1937). It is obvious therefore that any 

 substance found in the fluid may have had its origin in 

 the host and is not necessarily derived from the metabolic 

 activities of the parasite. It is regrettable that the gas 

 contents of the hydatid fluid have so far never been 

 studied; it would be of interest to know whether larger 

 amounts of oxygen enter the cysts by diffusion. 



IV. ECHINODERMS 



No echinoderm has so far been reported from oxygen- 

 free surroundings. This might be due to poorh^ devel- 

 oped anaerobic functions in all members of this phylum, 

 or possibly to the lack of investigation in that direction. 

 It is common knowledge, however, that many sea urchins 

 and holothurians bury themselves in the sand and mud 

 of the seabottom. If it is true, as was pointed out pre- 

 viously, that the bottom material of the sea is frequently 

 devoid of molecular oxygen, the echinoderms which pen- 

 etrate deeper into the bottom should be exposed to se- 

 vere anaerobic conditions. Whether they have means, 

 in all cases, of securing oxygen from the overlying water 

 layers remains to be seen. 



Echinoderms have not been subjected to much experi- 

 mental study, at least not in the adult stage. We know 

 only the work of Baglioni (1905) who showed that Echiuiis 



