4 The Microscope. 



margin bears four tufts of loug, tactile and vibratiJe hairs, which 

 the animal can move singly or together in the cluster. On the rear 

 part of the neck, and on the posterior portion of the back near the 

 caudal furcation, there are four additional hairs, two on each of 

 these parts standing perpendicularly to the cuticular surface, but 

 apparently without vibratile power. They are presumably tactile, 

 and have not, so far as I know, been previously observed. They are 

 present in every species that has come beneath my notice. They 

 are shown in Plate I, figures 2, 8 and 9, and in Plate II, figure 21 ; 

 from the other figures they have been omitted. 



The back and sides are variously armed with scales, hairs, 

 spines or pricks, and on some individuals with both scales and 

 spines. Ehrenberg instituted the genus Icthydium for that form 

 agreeing in structure with Chcetonotus, except that it has no hairs 

 or other appendages on the back. The genus contains but one 

 species, Icthydium podara, probably Joblot's fish with the trefoil 

 head, which occurs sparingly in New Jersey fresh waters. Ehren- 

 berg' s description is : Posterior extremity forked, body without 

 hair; and he states that in one instance he observed a single ventral 

 band of cilia, while in others he could find none. If the system- 

 atist considers this a valid genus, then four of our American forms 

 belong to it, a disposal that I should regret, and the correctness of 

 which I should doubt. The four forms referred to are Ch. loricatus 

 (Plate II, figui-e 6); Ch. rhomboides (Plate II, figures 31-35), where 

 the dorsum of each is clothed with scales; Ch. concinnus (Plate I, 

 figure 6), where the back and sides are entirely covered by hemi- 

 spherical papillae; and Ch. sulcatiis (Plate I, figure 15), in which the 

 same parts are sulcate in transverse furrows. Habits, internal 

 structure and mode of reproduction are essentially similar to those 

 of Chsetonotus. The body hairs are represented in them all by the 

 four dorsal bristles hitherto overlooked. 



The caudal prolongations in all the species are flexible and 

 movable. They have an interesting function, that of anchoring the 

 creatm'e to the glass slide or to some support in the water while the 

 Chfetonotus feeds. They are slightly enlarged distally, the center 

 of each being partially occupied by the duct of an ovate, gland-like 

 organ situated at their anterior end just within the body proper. 

 These can be seen in Plate I, figures 2, 6 and lid; from the other 

 figures they have been omitted. It is supposed that the secretion of 

 the glands is adhesive and assists the animal in clinging to its 

 support. Prof. Fernald in his paper already referred to says : " It 



