50 CHAETODERMA HAWAIIENSIS. 



Tho cuticular plate covering the fi'oiital sense organ is almost circular in 

 outline and is elevated above the general surface of the prothorax. This is 

 especially the case with the lateral portions which assume the form of pronounced 

 folds decreasing in height as the centre of the organ is approached. The boun- 

 dary between the cuticle and the underlying epithelial cells is not sharp and the 

 outlines of the cells themselves are not clearly visible. The greater number 

 appear to be sen.sory elements and are distinguished by their relatively slender 

 appearance (the diameter being to the height as one to four) and by darkly 

 staining elongated nuclei placed in the basal half of the cell. Among these are 

 a few cells of the same height, but of greater diameter, which contain spherical 

 centrally placed nuclei with a small amount of chromatin. Great numbers of 

 ganglion cells are situated in immediate contact with this sensory plate, and some 

 of the more deeply seated clearly connect with nerves passing out from the 

 precerebral ganglia (as I have termed the great accumulations of ganglion cells 

 in contact with the brain in the Chaetodermatidae) , and on the other hand 

 originate fibres that pass down to the frontal organ (Plate 28, fig. 1). A very 

 few small pyriform gland cells, staining almost black in haematoxylin, extend 

 from the midst of the ganglion cells to the sense organ where they probably open 

 to the surface. Large numbers of muscle fibres attach also to the sensory epi- 

 thelium and at several points there are indications that they pass between the 

 hypodermal cells and connect directly with the overlying cuticle. 



The hyjiodermis is i)ractically the same as in other species antl not jjarticu- 

 larly favorable f(or the solution of any of the several problems connected with it. 

 Its cells differ considerably in form and appearance. In th(> swollen jsart of the 

 prothorax they are slightly higher than broad; in the metathorax the reverse 

 is true; while in the abdominal region the height is about twice the diameter. 

 Everywhere their boundaries are indistinct, and thus unlike the sharply defined 

 central nuclei. Here and there are more slender elements with elongated darkly 

 staining nuclei, and somewhat more numerous are the basal cells in contact 

 with the base of the overlying spicules. These latter cells vary widely in general 

 appearance from very small compact elements to others large, globular, and much 

 vacuolated, owing to different stages of development and probably to some 

 extent to mechanical compression. No pigment cells exist, nor wandering cells 

 nor other elements that are sufficiently different from the usual constituents to 

 be jjlaced in a separate catagory. 



The spicules are of the usual spearhead shape, and form a continuous 

 series of increasing size, from those of the prothorax with a length of .0275 mm. 



