28 AllT. 7. — I. IJIMA : HEXCTINELLIDA, IV. 



hexasters in the periphery and those in the deep part of tlie wall. 



The former, as for instance those situated in the subdermal 

 space, are as a rule normally developed, showing generally o, 

 but sometimes 2 or 4, terminals to each principal. The terminals 

 are thin, measuring only about 2 /^ in thickness at base. 



The latter, as represented by those in the gastral layer, have 

 perceptibly stronger terminals, about 3 !>- thick at base. More- 

 over they are but seldom normally developed, exhibiting 2 ter- 

 minals to each principal. The majority are hemihexactinose, 

 there being also found not uncommonly quite hexactinose forms 

 (PI. II., figs. Ö, G). Of the hemihexactinose forms, the total 

 luimber of terminals to the entire rosette may vary between 

 7 and 11 (most frequently 1) or 10) indicating in each 

 case the number of the principals wliicli remain l^iterminal and 

 of those which had become unitermiual. In some cases one 

 of the two terminals borne on a principal was found to be very 

 much shorter than the other, these shorter terminals representing 

 without doubt intermediate stages in the transition from a biter- 

 minal principal into a uniterminal. In both the hemihexactinose 

 and hexactinose forms, the uniterminal principal may show the 

 well-known bending at the point of its junction with the single 

 terminal remaining to it, the two parts being not straightened out 

 as they quite frequently are. 



2. JJiScohexaster. Much less numerous than the oxyhexasters 

 are the discohexasters (PI. II., fig. 7), which occur subdermally 

 and somewhat less sparingly in the choanosome. In a few cases 

 they were found together with oxyhexasters on the free proximal 

 ray of gastralia. Diameter, DO-loO /'. Each short principal, 

 which is swollen knob-like at end, bears 4-6, long, slender, 

 faintly rough-surfaced terminals diverging so as to give an a2)prox- 



