246 I. IJIMA : HEXACTINELLIDA. I. 



sieve-plate of about 8 mm. diameter. The cuif is as yet uarrow. 

 Of the spiculation, I need mention only the fact that among the 

 hexactin-dermalia are here and tliere intermingled the same 

 pentactins, characterized by the peculiar distribution of micro- 

 tubercles OD the rays, as those which were found in the dermal 

 layer of all the smaller specimens. 



The pentactin-dermalia were not noticed by me in the 

 adults. 'With advance in age, they are either lost or become so 

 outnumbered by the multitude of hesactiu-forms as to easily 

 elude being seen. 



From the data presented in the above, it may not be in- 

 appropriate to draw up the following summarizations. 

 Firstly as regards the macroscopic features : 



H. olcinoseana, in the first stage of postembryonal de- 

 velopment, has a smooth and imperforate lateral wall, and 

 is provided with a single terminal osculuin. Soon (when the 

 body measures, say, 13-15 mm. in length) the outer surface 

 becomes uneven, and this stage is followed by the appear- 

 ance of parietal oscula. Later (say, when over 20 mm. in 

 length), the sieve-plate seems to become started by ad- 

 dition of new terminal oscula to the one already present. 

 The initial number of the meshes must at all events be very 

 few. Individuals, say, 50-70 mm. long, show the cuff and 

 ledges indistinctly developed. 

 Secondly, with respect to spiculation : 



The initial dermalia are pentactins.'-' To them, hexac- 



*Tliis remarkable fact is perhaps to be explained by assuming that at tlie time of the 

 first appearance of the dermalia the superficial trabecular laj'er isofsucli limited thickness 

 that it aflbrds no space for tlie development of a siitli distal ray, the paratangentials being 

 formed on its extreme outer surface. A plienomenon in a measure analogous to this is seen 

 in the development of Laicupsacus ortliodocus- In tliis species the first spicides formed in the 



