504 W. WOODLAND. 



Throretical. 



With regard to the cause or causes determining tlie form 

 of the Auriculariau spicules, little cau be said. It seems 

 probable that the flattened disc- and later cup-forms are at 

 least in part due to contact of the spicule with the body-wall, 

 unless these ai-e indeed to be attributed to the presence of the 

 nuclei on the inner side of the plate only, the nuclei, contrary 

 to the usual assumption, preventing deposition in their 

 immediate vicinity.^ 



But whatever forms the spicules may assume, it is clear that 

 one of their inevitable attributes, viz. weight, may play an im- 

 portant part in the life of the larva. In the Auricularia3 1 have 

 studied, the wheels and globes are all situated, as before stated, 

 at the lowest extremity of the larva, and it seems certain that 

 this extremity is lowest because of the presence of the 

 relatively weighty spicules in this region. In other words, 

 the larva maintains a certain position in the Avater because it 

 is appropriately weighted. The so-called baguettes de 

 corps of the pluteus larva doubtless serve a similar 

 function. 



Summary. 



(1) The spicule first appears as a granule contained in a 

 syncytium, in which, however, the scleroblasts retain their 

 individuality to some extent, which is not the case in the 

 syncytium of Synapta. 



(2) The spicule becomes disc- and then cup-shaped, 

 develops the spokes as outgrowths from the margin of the 

 disc, and finally forms the felly of the adult wheel, the spicule 

 during the whole of its development being enclosed by the 

 syncytium in which all the nuclei (scleroblasts) are situated 

 on its internal side, i.e. away from the body-wall against 

 which the spicule lies. 



' I may point out in this connection ti:at llerouard (3) attributeil, tliough 

 quite erroneously, the perforations of the liolotlmrian phite to the presence 

 of nuclei. See Study IV. 



