34 ARKIV FÖR ZOOLOGI. BAND 13. N:0 25. 



Stichopus tremula. 



Two types of crystals are present: biconvex rhomboids 

 of the ordinary type, measuring up to 15,2 \l in length, and 

 stars with six short rays, the diameter of which amounts to 

 11,4 [i. Viewed from the side, the stars present themselves 

 as cubes (pl. 5, figs. 2 — 3). 



Mesothuria intestinalis. 



As far as I could make ont, only one kind of crystals 

 is present, viz. rectangular rods or quadrangular prisms. 

 When young and small the prisms are lodged in globular 

 mother-cells which change form during the increase. The 

 length of the prisms amounts from 7,6 \l to 15,2 [i (pl. 5, 

 figs. 19 and 20). To judge from the figures of Semper^ simi- 

 lar prisms also occur in Holothuria coluber from the Philippine 

 Islands. It may possibly be proved in the future that such 

 prisms are characteristic for most of the forms which belong 

 to the genus Holothuria and its nearest relatives. Now and 

 then I have seen the plasm of the mother-cells exhibiting 

 amoeboid movements by protruding pseudopodia. 



Retrospect. 



1. The crystals cannot be looked upon as products of 

 chemical or physical forces alone. They cannot be crystals 

 in common sense of the word. They have arisen entirely 

 under the influence of living organic substance, and their 

 existence is totally dependent on vital forces. 



2. The mode in which the crystals take their orgine and 

 the stränge appearance of some of them let us presume that 

 they ought to be looked upon as spicules, in several respects 

 equivalent to those which occur in the skin of certain holo- 

 thurids and especially to the spicules of certain larvse of echi- 

 nids. 



3. From that mentioned above, we may be entitled to 

 presume that the skeleton of the echinoderms in common 

 is due to the activity of migratory plasma-amoebocytes and 

 their syncytial fusion. This presumption is supported too by the 

 bewildering resemblance which exists between the structural 



^ Op. cit. 



