246 I. IJIMA : HEXACTINELLIDA. I. 



sieve-plate of about 8 mm. diameter. The cuff is as yet narrow. 

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

 hexactin-dermalia are here and there intermingled the same 

 pentactins, characterized by the peculiar distribution of micro- 

 tubercles on tlie 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 hexactin-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 : 



R. oHnoseana, in the first stage of postembryonal de- 

 velopment, has a smooth and imperforate lateral wall, and 

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

 body measures, say, 18-15 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- 



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

 first appearance of tlie dermalia tlie superficial trabecular layer is of such limited thickness 

 that it affords no space for the development of a sixth distal ray, the paratangentials being 

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

 in tlie development of Leucopsacus ortliodocus- In this si)ecies the first spicules formed in the 



