Ttvo Peculiar Actinian Larx'cr from Tortugas, Florida. 179 



although all of the macrosepta are well-developed and the niicrosepta are 

 already present. In the oldest of these larvre, 8 mm. in lenp;th, the pharynx 

 is only i.i mm. long. In cross-section the pharynx of Zoanthclla is quad- 

 rilateral in shape, the four sides of the quadrilateral being incurved toward 

 the center (plate 2, fig. 3). The longitudinal plications of the walls of the 

 pharynx correspond in position to the primary septa (macrosepta) and below 

 the pharynx these plications are directly continuous with the mesenterial fila- 

 ments of these septa (plate 3, figs. 4-6). 



The septa consist of three pairs of macrosepta and three of niicrosepta, 

 and, as Van Beneden has shown, their arrangement and sequence corresponds 

 to that of the Zoanthea; (microtype of Erdmann), /. c, the dorsal directives 

 are imperfect, the ventral are perfect, and of the two remaining pairs on each 

 side the dorsal member of each pair is perfect and the ventral one imper- 

 fect. Van Beneden holds that the order of appearance of the septa, as 

 judged by their size, is for the macrosepta: 1, lateral; 2, dorsal; 3, ventral; 

 and for the niicrosepta : 4, dorso-lateral ; 5, ventro-lateral ; 6, dorsal direct- 

 ives. In the youngest ZoantheUa which I have sectioned the dorsal macro- 

 septa are smaller than either of the other pairs (plates 2 and 3), and judged 

 by the standard hitherto used they are the last-formed of the macrosepta. 

 Three pairs of microsepta are present in these youngest larv^e and they are all 

 about equal in size. 



The macrosepta are triangular in cross-section, as shown in plates 3 and 

 4 and text-fig. 4, the base of the triangle lying at the central border of the 

 septum. To this thickened central border the cylindrical mesenterial fila- 

 ments are attached. A similar condition was observed by Heath in Zoan- 

 theUa galapagoensis, but Van Beneden does not figure or describe it in the 

 other species of this genus. 



In the interspaces between macrosepts and niicrosepts are endodermal 

 thickenings, which are sometimes as prominent as the septa themselves. 

 These thickenings consist of vacuolated endoderm cells and in the younger 

 larvae (plate 3) they fill up a large part of the cavity of the coelenteron, 

 especially at the aboral end; in the older larvae (text-fig.) they are much 

 less voluminous, probably representing the remains of the primitive endo- 

 derm which, in earlier stages, filled the entire enteron. 



Zoanthina. — The shape of this larva varies greatly with different stages 

 of contraction; it may be elongated as shown in plate i, figure 2, or con- 

 tracted so that the oral-aboral axis is no longer than the transverse one 

 (plate I, fig. 2a, text-fig. i). In some specimens the oral portion of the body, 

 in front of the ciliated band, is relatively much larger than in the specimen 

 represented in plate i, figure 2. In general this larva is shorter and stouter 

 than ZoantheUa. In color it resembles the latter, being of a light brownish- 

 yellow, mottled with darker spots ; some of the Xassau specimens are of a 

 violet tint. My smallest specimen of this genus is about i mm. long and 1.2 



