280 ME. J. E. DUEEDEN ON 



all my preparations reveal tlie curious fact that, in the process 

 ■of sectionizing, many of tlie nematocysts become dislodged from 

 the ectoderm, and dragged to what would be the margin of the 

 paraffin block, without causing any apparent injury to the other 

 elements. 



Here and there throughout the layer, more numerously in 

 the tentacular region, occur nematocysts in various stages of 

 development. They are at once recognized by their homogeneous 

 and deeply-staining character. In this respect they contrast 

 strongly with the mature forms, the vfalls of which are not 

 affected by borax-carmine, though they become slightly blue in 

 indigo-carmine. Most of the nematocysts originate in the deeper 

 parts of the layer, and, at first, are a little concave or irregular 

 in outline, and arranged obliquely or even tangentially to the 

 surface. When mature they extend as far as the external sur- 

 face and are disposed vertically. Both the wall and the axis 

 take up the stain in the nearly ripe condition, and in trans- 

 verse sections of earlier stages the contents are homogeneous in 

 the middle but granular towards the margin. 



Around the aboral end the ectoderm is of a very different cha- 

 racter from elsewhei'e (Pi. 19. fig. 21). The periphery is crowded 

 with small nematocysts, provided with a very distinct spiral 

 thread ; large, pyriform, deeply-staining bodies, perhaps glan- 

 dular, occur more internally, and towards the supporting lamella 

 the nervous layer is exceptionally well-developed. Such a special 

 development of the protective, sensory, and probably glandular 

 elements of the layer is, no doubt, in some way associated with 

 the fact that in the free active stage this end of the larva is 

 foremost, and that it is by this that it ultimately attaches 

 itself. McMurrich (1891, p. 317, pi. xiii. fig. 13) describes and 

 figures a somewhat similar exaggerated development of the 

 nervous layer in the embryo of 'Rhodactis. 



As already mentioned amongst the external characters, and 

 contrary to the usual condition in adult Zoantharia, the ectoderm 

 contains zooxanthellae, restricted, however, to the central area 

 of the oral disc. In the oldest larvsa they occur throughout the 

 thickness of the discal ectoderm, but none remain in the adult 

 polyps. 



Compared in other respects with the ectoderm of the adult, 

 there is a great increase in the latter in the proportion of uni- 



