47 



has a slight sinuation in the middle. Over the whole body, as well as 

 along the vibratile band are scattered red-brown pigment cells; in the 

 end of the arms and in the posterior end of the body they may be lying 

 somewhat closer, but there is never a very prominent pigment spot in 

 the end of the arms. — ■ The skeleton is simple. There is no basket-structure 

 and not even an indication of a recurrent rod. The body rods are club- 

 shaped, with some irregular, short branches at the thickened end. Other- 

 wise the skeletal rods are very slender and very finely thorny. (Fig. 12). 



At the age of ca. 

 3 weeks the postero- 

 dorsal arms had be- 

 gun to appear, their 



supporting rods 

 being also simple 

 and very slender. At 

 the age of one month 

 the posterior end of 

 the body rods showed 

 signs of beginning 

 absorption; the dor- 

 sal arch had ap- 

 peared and the pre- 

 oral arms had begun 

 to develop. A small 

 vesicle lying opposite 

 the hydrocoel on the 

 left side evidently 

 represented the be- 

 ginning amnion-for- 

 mation. At five weeks the body rods had been almost completely ab- 

 sorbed. There was otherwise no change — no posterior transverse rod 

 and no epaulets. The larvae still lived for nearly two weeks more, but 

 they did not develop any further. The question whether epaulets are 

 present in the fully developed larva of this species thus still remains 

 open. No new culture of this species could be obtained at that time, its 

 breeding season being over. 



Fig. 12. Skeleton of the larva of Strongylocentrotus pulcherrimus. 



1st stage. A. ventral view, B. Side view, ^""/i- Letters as in fig. 5. 



vtr. ventral transverse rod. 



The larva of Str. drobachiensis was reared by Agassiz till the age 

 of 4 weeks. The postero-dorsal arms and the dorsal arch had appeared, 

 but apparently he has not observed any beginning absorption of the body 

 rods. As for the later stages described and figured by Agassiz as be- 



