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it will be seen that the exposed, cilia bearing portion is of much smaller 

 extent than the side in contact with the mesogloea and is therefore 

 specifically different from Z. hcnseni where the ciliated cells in cross 

 section form a very broad, almost rectangular plate. 



As van Bene den states for Z. henseni each cell is exceedingly 

 slender with the usually elliptical, deeply staining, compact nucleus 

 placed at a varying distance from the basal end. The distal fourth of 

 the cell is finely granular, almost homogeneous, and is bounded exter- 

 nally by a delicate perfectly distinct cuticular border (Cuticularsaum) 

 surmounted by a tremendous cilium. In Z. senipcri the cilia are fused 

 into a vibratile plate which persists even in preserved material. With 

 Z. galapagoensis , whatever may have been the case in life, there is no 

 sign in preserved material of a fusion to form an undulating membrane 

 and the same appears to be true of Z. henseni. This last named species 

 is also peculiar in having the ciliated plate formed "d'une seule et même 

 espèce de cellules" while in the present species members of pigment 

 cells are present in the deeper portions and often extend their delicate 

 processes for varying distances toward the outer surface of the body. 



The general ectoderm consists of [a] the usual slender elements 

 together with occasional more spindle shaped bodies having much the 

 appearance of sensory cells, [b] gland cells, [c] nematocysts and [d] pig- 

 ment cells. The last named occupy the deeper portions of the layer and 

 are apparently stellate in form, the slender processes often extending 

 in various directions for considerable distances. The nematocysts, as 

 van Bene den has noted in Z. henseni, are of two distinct types, one 

 relativly small and slender and very abundant form with one end level 

 with the surface of the body; and another rarer, larger and of more 

 elliptical shape similarly located or irregularly placed in the vicinity 

 of the mesogloea where they undergo all but the last stages of their 

 development. The gland cells appear to belong to one class. Barely 

 one meets with comparativly large cells filled with a homogeneous darkly 

 staining secretion, but in some cases, at least, they appear to be degene- 

 rate nematocysts, or compound structures due to the union of typical 

 glandular elements. These latter are usually very slender and vase- 

 like, often spindle shaped, with an intensely staining secretion in the 

 distal half, while the nucleus holds a position between this and the deli- 

 cate stem. 



As in other species belonging to the genus there are no tentacles 

 about the mouth, but owing to the presence of numerous particles of 

 organic debris entangled among the ciha it is possible that the ciliated 

 plate, in addition to acting as a locomotor organ, may also serve to drive 

 microscopic organisms or organic particles into the mouth. On the other 



12* 



