Tzuo Peculiar Actinian Larvcc from Tortiigas. Florida. iSi 



ivcs are imperfect, the ventral are i)erfcct. and of the two remaining pairs 

 on each side the dorsal member of each pair is perfect and the ventral 

 one imperfect, while the order of appearance is, for the macrosepta, i, 

 lateral ; 2, dorsal ; 3, ventral ; and for the microsepta, 4, dorso-lateral ; 5, 

 ventro-Iatcral ; 6, dorsal directives. 



McMurrich agrees with Van Beneden that the arrangement of the septa 

 of Zoanthina is characteristic of the Zoanthccc, but he suggests that the 

 sequence observed by Van Beneden is really due to the retardation of the 

 development of the dorsal directives, which should, according to his view, 

 stand fourth in the order of development. 



In all my sections of Zoanthina the dorsal directives are smaller than 

 the other microsepta (text-fig. i and figs. 24, 25), and if the order of appear- 

 ance is to be judged by the relative sizes of the septa, the dorsal directives 

 are the last to appear of these 6 pairs. Therefore, my observations as to the 

 sequence of the septa in Zoa/iZ/nVu; agree with the conclusions of Van Beneden. 



The individual septa are not triangular in cross-section, as in Zoan- 

 thella; on the contrary they are nearly as wide at the base as at the free 

 border, and they all show a greater thickening on one side of the mesoglcea 

 than on the other (figs. 24, 25 and text-fig. 1). In the ventral and dorsal 

 directives these thickenings face the median plane, in the laterals and ventro- 

 laterals they face the ventral side, in the dorso-laterals and dorsals they 

 face the dorsal side. 



.Although the microsepta are well-developed, the macrosepta only are 

 united to the pharynx (fig. 21). Van Beneden found that the septa in the 

 oral and aboral portions of the body were not continuous through the region 

 of the constriction. In the Tortugas specimens the septa are small in this 

 region, but they are not interrupted (figs. 22 and 23). Sections through 

 the outer fold of the ciliated groove show the presence of 12 pockets, formed 

 by the 12 septa (fig. 23). 



HISTOLOGY. 



In most respects the histological character of these two types of larvae 

 is similar, though there are certain minor differences. 



Ectoderm. The ectoderm consists of greatly elongated cells, among 

 which are numerous cell-spaces. At the free border of the epithelium no 

 cell-spaces are visible and the cells are here more darkly stained than at 

 deeper levels. The inner ends of the cells are narrow and apparently few of 

 them run through the entire epithelium from the free border to the meso- 

 glcEa. Among the epithelial cells are nematocysts of two types, smaller ones 

 which lie near the surface, in fact with one end of the nematocyst at the 

 free border of the epithelium, and much larger ones which lie at a deeper 

 level. In text-figure 2. which is a section through the ectoderm and meso- 

 glcea of Zoanthella, both kinds of nematocysts are shown and at the base of 



