MOEPHOLOGY OF CYCLOPS. 33 



clevelopracnt iii the male is au argument ofteu used iu support of their oliactory 

 chai'acter. 



The pore-canals (described on p. 20) are probably also sense-organs. In support 

 of the aiuUton/ character I have ascribed to concretions found at one time constantly 

 at the base of tlie fifth thoracic appendage of the male of C. hremcornis, I cite 

 an observation of Clans*: — "Possibly a peculiar structure iu the brain of C((lu- 

 nella belongs to the category of auditory organs. There are two spherical spaces, like 

 otocysts (Taf. vii. fig. 9), in whose clear contents a ball of concretions is visible. 

 Whether this differentiation be constant or not, I have unfortunately been unable to 

 determine." My concretions occurred singly, or in an aggregate of two or tlirce, in a 

 little clear vesicle on a vacuolated mass lying on the outside of the ganglionic dilatation 

 of the nerve, just below the base of the limb. I believe that the vacuolated mass Avas 

 the pore-canal which lies in this position, but my knowledge then was insufficient to carrv 

 me so far. The very inconstancy of these organs is, strangely enough, a point of identity 

 with Claiis's suggested otocysts ; and tliat they should occur in tlie pore-canal cell, 

 instead of in the brain, is by no means an unparallellecl occurrence. Indeed, if we admit 

 their auditory function, we get a side-light on the meaning of the two-fold condition 

 (closed and open) of the auditory organ of the Malacostraca. It may well have arisen 

 from the confluence of a number of pore-canal cells, perhaps originally sunk in a special 

 pit. I offer this as a suggestion to others. 



Sense-Organs. 



Bye (PI. II. figs. 7, 8 ; PI. IV. figs. 4, 0, 7, S, 15).— The eye of Cijclops is situated in 

 the frontal region, resting immediately on the anterior end of the brain. In brief, it 

 consists of three spheroidal ocelli, two lateral and an infero-median (the latter, as usual, 

 first detected by Glaus), imbedded in sockets lined with pigment in a central sup])orting 

 mass. Each ocellus is composed of a number of bluntly fusiform bacilli, placed radially 

 and containing a nucleus distal to their centre ; the ocellus receives the very short optic 

 nerve posteriorly at its outer surface, so that the optic elements are reversed, as in 

 the eye of Dendrocoelum lacteum-f. The central mass is divided into three blocks by 

 fine membranous partitions — one superomedkm sagittal, separating the blocks which 

 receive the two lateral ocelli, and one iiifero-horizontal separating these from the 

 block for the inferior ocellus. A similar posterior partition separates tlie central 

 mass from the brain, and is quite imperforate by nerves. Each of these blocks contains 

 at least one nucleus, probably two, an anterior and a posterior. The tapetum consists 

 of fine reddish granules, lying on the face of the block, and giving a brilliant metallic 

 lustre by reflected light or dark-ground illumination. 



The median ocellus is nearly spherical, containing about eight peripheral and one 

 central bacillus. It is connected with the rostrum by two fine slips of fibres, diverging 



* Freil. Cop. p. .5(i, Taf. vii. fig. 9. 



t See the paper by Justus Carriere, Archiv f'iir raikros. Anat. 1S82. Sar/illa has. according to the Hertwigs, a 

 pair of similar eyes, each of three inverted ocelli with an internal " lens," possibly eijuivalenl to the " block."' 

 SECOND SEEIES. — ZOOLOGY, VOL. V. 5 



