116 



I have observed in some sections but they appear to be merely a casual 

 arrangement of the nuclei. On the other hand 1 have found in a series of 

 transverse sections of a specimen, 41—43 hours old, a distinct canal pro- 

 ceeding from the enterocoel and opening on the left side, at the level of the 

 amnion (PI. XVI, Figs. 19—20). This canal, the existence of which is un- 

 disputable, would rather seem to represent the pore canal, in spite of 

 its unusual position. But in order to ascertain the correctness of this 

 interpretation it would be necessary to follow its further development 

 and see the pore assume the normal position at the aboral pole of the 

 urchin; this, however, could not be done on the material available. The 

 question of the development of the hydropore (and stone canal) thus 

 cannot be regarded as settled. On the other hand I must ascertain that 

 no trace of a hydropore is observable in the young larva. 



It is a noteworthy fact that the larvae of 29 hours age studied by Miss 

 Medes were in a considerably more advanced stage of development than 

 those of the corresponding age in my material; the spines and tubefeet 

 have not reached the same stage of development here until the age of 

 ca. 50 hours. This may depend on the temperature, the developmental 

 processes going faster in the tropical conditions of Badu Island than in 

 the cooler chmate of Japan. Possibly it may also indicate that the Japanese 

 form is not really the same species as the tropical form. — By this time 

 the tubefeet and spines begin to protrude from the amniotic cavity. This 

 takes place partly through the natural opening, which has gradually been 

 somewhat transplaced to the dorsal side of the larva (PI. XVI, Fig. 10; 

 comp. also figure 20 of Miss Medes' paper), the young sea-urchin thus 

 in fact coming out of the mouth of the larva; partly also the dorsal wall 

 of the amnion, together with the dorsal wall of the 

 larva is broken through directly, both the dorsal wall 

 of the amnion and the larval skin being by that time 

 very thin. 



The development of the plates and other structures 



of the young sea-urchin cannot be considered here. I 



would only point out the presence of a pair of long 



spines at the posterior end of the body, much as it 



is represented in Echinocyamus pusillus in PI. IX, 



Fig. 107 of Theel's monograph. Such a pair of long 



posterior spines were also found in the newly meta- 



Fig. 48. Larva of Pero- Hiorphoscd Astriclypeus manni, and they may per- 



neiia Lesueuri, 5 days haps prove to be of general occurrence in young Cly- 



inj 2Z7%:rZe P^astroids (and Spatangoids) ; possibly they may have 



band. i"»/i. some phylogeuctic meaning. 



