258 The Microscope. 



whose presence permits the escape of the sticky mucus or 

 slime secreted by the ectoderm cells. The high power applied 

 to vary their sections shows the vertical elements, barely visible 

 under the low power, to be nucleated, cells tall and narrow and 

 standing closely packed. We remember that animal cells are 

 thin, delicate-walled, therefore we do not expect to see the indi- 

 vidual cells clearly outlined, but the closely-placed nuclei and 

 the lines which are parallel with them, crossing the layer every- 

 where, are indications which permit only one interpretation of 

 the shape of the cells : they must be columnar. There arc two 

 deviations from this type of cell in the layers of the ectoderm ; 

 one is the " goblet cell," seen here and there in any portion of 

 the skin. It is an oval, clear spot, which looks as if it might 

 have been the position of a former cell, now empty and gone. 

 The goblet cell is understood to be a cell of the ectodermal 

 layer, which has been caught at a moment when it is full of 

 mucus, just prior to discharging it through the pores of tlie 

 cuticle. The other kind of ectoderm cell is that lining the lower 

 portion at least, and perhaps all of the pit in which the seta is 

 lodged. In this position the cells are not columnar, but cubical, 

 as indicated by the shape and relative number and position of 

 the nuclei. The seta is a structureless, blunt bristle, similar 

 chemically and in appearance to the cuticle, and, like it, non- 

 cellular. 



The remaining thickness of the skin is chiefly muscular. The 

 individual muscle cells can be best seen in the circular muscle 

 layer, where, in cross-sections like the one we are studying, they 

 are seen lengthwise. The fibre thus displayed is parallel-sided 

 or nearly so — the long, deeply-stained body is its nucleus. The 

 round nuclei scattered about are those of the connective tissue 

 layer, the fibres of which can be seen filling in spaces where the 

 epithelium or muscular tissue may be wanting. The muscular 

 fibres of the longitudinal layer are cut endwise in one section. 

 They are very regularly arranged inside a framework of con- 

 nective tissue, which gives them a very regular appearance, as 

 shown at L. mu., in Fig. 2. This muscular tissue is somewhat 

 like that which is called involuntary or unstriped muscular 

 tissue in human anatomy ; it is the human histology, but is the 

 only kind in Lumhricus. Besides the circular and longitudinal 

 muscular layers, there are small muscular fibres, which are 



