96 BLIND VERTEBRATES AND THEIR EYES. 



in the alimentary canal. The opening of the oviduct has moved forward until 

 it lies in front of the pectorals and it has carried the anus forward with it. In 

 newly hatched individuals the anus has its normal position behind the ventrals. 

 When the fish has reached a length of 25 mm., the anus has reached a point in 

 front of the ventrals, but it is still nearer the ventrals than the pectorals; with 

 a length of 35 mm. the anus has moved forward to just below the insertion of 

 the pectorals. In mature specimens it lies considerably in advance of the pec- 

 torals (plate 5, fig. c). The forward movement of the sexual orifice takes place in 

 both sexes. 



Nothing is definitely known of the advantages of the location of the opening of 

 the oviduct. They can be inferred from the habit of Amblyopsis in carrying its 

 eggs in the gill cavity. Located as it is, the oviduct may be covered by the gill 

 membranes of the 2 sides alternately, or, if the fish takes an oblique position in 

 the water with the head down, the eggs may flow directly into the gill cavities, 

 being carried downward by gravity and held in the groove in front of the anus 

 by adhesion. 



It is difficult to imagine even a formal explanation of the origin of the position 

 of the sexual orifice in the Amblyopsidae. The anus was probably carried forward 

 as the result of the forward movement of the sexual orifice, and it is this that 

 demands explanation. Very probably the habit of carrying the young in the gill 

 pouches antedates the present position of the anus. The eggs may have been 

 allowed to flow into the gill openings, the female occupying a position with head 

 downward during oviposition. If this were the case, then, while the individual 

 skill would count for much in transferring the ova, a variation or mutation which 

 lessens the distance between the sexual orifice and the gills would be of distinct 

 advantage and would probably be transmitted by natural selection. The actual 

 transfer of the ova into the gill cavity has not been observed. 



THE TACTILE ORGANS. 



The tactile organs are among the most important in the consideration of the 

 blind forms. Their minute structure will form the basis of a separate paper. 

 The prominent tactile organs about the head of Amblyopsis have been mentioned 

 by nearly every writer, and they have been figured by Putnam-Wyman and Leidig ; 

 but the figures of the distribution of the ridges are worthless. The description 



by Professor Forbes of Chologaster papilli- 

 ferus is the only systematic enumeration of 

 the ridges that has appeared. The accom- 

 panying figures (32 and 33), drawn by me 

 with the camera lucida, verified and copied 



Fig. 31. Tactile Organ in Head of Larva: A mblyopds. See r^ t\/t tj r\ P-v_ mVo tViA pyort pvtpnt 

 also Plate 70 just above Yolk. Larva was placed Dy MX. U. KJ . l^OX, give ine exact eXltni 

 in weak osmic acid which brought out outlines i ... r^l "J • A t; t ' 



of structure. 8 mm. + 4 ocular. and position 01 the ridges in Amblyopsis, 



Typhlichthys, and Chologaster papilliferus. 

 It will be seen that in the number and distribution of the tactile area the three 

 forms agree very closely, the eyed form having the same number and dis- 

 tribution of ridges or rows that the blind forms have. In C. papilliferus most 

 of the ridges are much less prominent than in the blind species, being sunk 

 into the skin. About the nose and chin, however, the ridges are as prominent 



