INTEENAL ANATOMY OF BDELI.A. 503 



these branolies. These nerves run above, and are distributed to, the genitalia, and possibly 

 to some other organs of the hinder part of the body. 



The thin pair of nerves from the sub-oesophageal ganglion is a very fine pair {nd) 

 arising from the brain about midway between the second and third legs ; it innervates 

 the large dorso-ventral muscles which are situated between these two pairs of legs ; I 

 found one branch to this nerve. I believe the nerve has not been observed before in 

 any of the Acarina. 



I was not able to find the fine, recurrent, median nerve which exists in Thyas. 



The Eyes. 

 (Figs. 26, 27.) 



There is not much to be said respecting these organs, because they do not appear to 

 vary greatly from the known types of simple eyes in other Arachnida ; they are, 

 however, more highly developed than those usually found in such Acari as are provided 

 with those organs. 



The optic nerve, as before stated, bifurcates slightly near its distal end— one branch 

 being in etfect a continuation of the main nerve, and the other being very short; each 

 branch expands at its end inio what I have called a retinal disc {rcl) ; this is an almost 

 circular plate, only very slightly cup-shaped—/, e., only slightly concave on the side 

 next the cuticle. These discs are composed of rather loosely packed ganglionic cells 

 and are more than one cell thick ; the exterior cells, next the rods, are very distinctlv 

 nucleated, the nuclei of the deeper cells being more irregular. Abutting on the outer 

 cells of the disc are a considerable number of rods or retinulac {re) converging to the lens ; 

 these rods have rounded posterior ends, and are remarkable for the almost entire absence 

 of pigment between and around them ; much more pigment is found in the eyes of such 

 Hydrachnidte as I have seen, although tlie rods there are not so distinct nor numerous. 

 'The smaller outer ends of the rods abut upon the lens {ol), which is bounded exteriorly 

 by the extremely convex cornea, W' hich is more compact and highly refracting than the 

 rest ol the lens. The two eyes forming the pair on each side are, from their situation, 

 turned in slightly different directions ; they are not as close together as in most 

 Hydrachnidse. 



The Eepeoductive System. 



Tlie JIale. (Figs. 17, IS, 19, 20, 21, 22, 23, 34, 35, 36, 37, 38, 39, 10, 11, 42.) 



The genital organs of the male Bdella have far more resemblance to those of 

 Tromhidimii JuUyinosmn, as described by Henkin, than to those of any other Acarid 

 of which these parts have hitherto been investigated ; indeed, his is the only drawing 

 and descrijotion which can be said to form any guide to the general arrangement of the 

 parts : still it is only in the general arrangement that this can be said. Henkin's 

 descri])tion is, I believe, quite correct in almost every particular. I have dissected 

 Trombidium Julicjiuosum and have much pleasure in confirming Henkin's work ; but 

 when we compare it with Bdella, we find that although the two types are more like each 



68* 



