5° 



INANITION AND MALNUTRITION 



Fig. 19. Fig. 20. 



Fig. 19. — Larva (abnormal pluteus) of the sea-urchin Echinus reared in a solution the 

 same as the preceding, but with addition of MgS04. Fourth day. Except for the absence 

 of the skeleton, the internal structure corresponds somewhat to that of the normal pluteus 

 stage (cf. Fig. 16); but the external form is very different, arms undeveloped, etc. The 

 addition of CaS04 to the medium permits normal development, indicating that the organism 

 requires this salt as a source of sulphur, that in MgS04 being inadequate. 



Fig. 20. — Abnormal pluteus representing the maximum stage of development reached by 

 the sea-urchin Echinus reared in an Mg-free mixture. Third day. No arm-processes or oral 

 invagination. Gut and skeletal development somewhat retarded. Indicates the necessity 

 of magnesium for normal development. 



Fig. 2i. Fig. 22. 



Fig. 21. — Abnormal pluteus of sea-urchin Sphaerechinus reared in a mixture containing 

 CaHP04, but not CaC03. Third day, showing the maximum stage of development reached. 

 The form is abnormal; the "snout-like" process and absence of skeleton being especially 

 characteristic in the absence of calcium carbonate. 



Fig. 22. — Abnormal pluteus of the sea-urchin Echinus, reared in mixture containing NaCl, 

 KC1, MgS0 4 , CaS0 4 , Ca 3 P20s and FeC03. Deficiency in CaC03 results in complete absence 

 of skeletal development, and in abnormal external form differing from that shown for Sphaere- 

 chinus in the preceding figure. Third day, representing the most advanced stage of develop- 

 ment reached. 



