ON DEVELOrMENT ETC. OF PlIOROXIS. OOÖ 



iiLiiiKTüUS, sometimes reaching 48 in niiiiiber. In ;■, single living 

 specimen, the skin of the trnnk was of a light orange colour ; the 

 sul »dermal circular muscles were especially well developed in the 

 trunk but intei'ruj)ted at four longitudinal clearly marked zones. 



The youngest swimming larva I have ever obtained was of 

 type A. It was already supplied with four pairs of tentacles 

 which, however, wTre still short. The l)ody measured about 0.5 

 //i/ii. in lençrth. The trunk was short and showed a sliirht 

 characteristic curvature, the concavity being turned toward the 

 dorsal side.^ The thickly ciliated hood was comparatively large; 

 the ganglion and the perianal ciliated belt were already well deve- 

 loped. In the surface view of this larva during life, I was not 

 aljle to detect the ventral pouch nor the c(jrpuscle-masses. 



At about the stage vvitli five pairs of tentacles, the trunk 

 becomes elongated and straightend out. The nephridia may then 

 be seen in their characteristic bouquet-foim, and the ventral 

 pouch appears as a solid ectoblastic thickening. Neither the 

 corpuscle-masses nor the retractor muscles are yet to he seen. 



As the larva grows, the number of tentacles increases in pairs 

 proceeding from the ventral side toward the dorsal ; hence, the 

 most dorsally situated tentacles are the youngest and the shortest. 

 In larVcC with 12 tentacles and Ijclonging to type A, the ventral 

 pouch is deep enough to l)e plainly visible from outside. We 

 always notice from this stage on a pair of the retractor nmscles 

 which extends Ijctween the ganglion {(//.) and the dorsal inner side 

 of the tentacular circle {ret. in figs. 12, lo, 14, and 1Ö). 



The larval organisation of types A and J> is nearly com- 

 pleted in the stages with 14-1(> tentacles. J^et me next give a 

 somewhat detailed description of the external aj)pearance of Actino- 

 troeha in ueneral. 



