168 DESCRIPTIONS OF PREPARATIONS. 



from a process of the procephalic lobes, and the eye is not stalked at its first 

 appearance in those forms of Crustacea in which it is stalked in the adult when they 

 pass through a complete series of developmental forms commencing with a Nauplius 

 or Metanauplius. The series of true appendages appears to commence with the 

 first antennae. 



The eye itself surmounts the terminal joint. It has a convex, soft and trans- 

 parent cornea marked out by faint lines into square facets. The visual structures 

 are arranged in two layers. To each corneal facet corresponds an eye-element, i. e. a 

 crystalline cone and a retinula. The former is derived from cells of the ectoderm 

 of the procephalic lobes, the latter from the supra-oesophageal ganglion (?). The 

 crystalline cone is formed from four crystalline cells. The nuclei of the cells and a 

 small quantity of protoplasm lie immediately under the corneal facet. The outer 

 part of the crystalline cone is less refractile than the inner part, which is long and 

 has the shape of a four-sided pyramid terminated by a long pointed piece which has 

 four projections fitting into the rhabdome of the retinula. The retinula is composed, 

 as in Palaemon, of retinal cells grouped round a rhabdome formed of four square 

 chitinoid rods. Each rod swells out posteriorly, is red in colour, and is marked by 

 alternate light and dark striae which do not correspond in position in adjoining rods. 

 The retinula cells surround the rhabdome and contain black pigment granules. A 

 basement membrane pierced by the nerve fibres, one to each retinula, separates the 

 retinulae from the optic ganglion. Nucleated yellow brown pigment cells intervene 

 between the retinulae, and two (or more) black pigment cells between the crystal- 

 line cones. The ectoderm cells at the margin of the eye are elongated and filled 

 with black pigment *. 



The protopodite of the first antenna is three-jointed. The basal joint is tri-hedral 

 and lodges the auditory sac. The exo- and endo-podite are both annulated. The 

 number of joints in the exopodite vary, but, distal to the first eight, they bear on 

 their ventral surfaces the olfactory setae so-called. The auditory sac is curved, and 

 possesses delicate chitinous walls. Its aperture, which is permanently open, but is 

 protected by numerous setae springing from its outer margin, lies in the dorsal 

 surface of the joint. The auditory setae are arranged in two rows which meet at the 

 closed end of the sac. The largest setae are one-fiftieth of an inch long. They are 

 hollow and are moveably attached to the wall of the sac. This attachment is by a 

 membrane -delicate on one side, stout on the other forming the tooth ' of Hensen. 

 A narrow plate, the ' ligula ' of Hensen, is developed in the shaft of the setae, on the 

 side opposite the tooth. The shaft with the exception of the ligula is beset with 

 five solid barbs. The nerve fibre is furnished with a ganglion cell close to its ter- 

 mination, which is fine and delicate, and according to Hensen attached to the 

 ligula. The otoliths are numerous and irregular in shape. According to Hensen's 

 observations on Palaemon antennarius, these otoliths are foreign particles collected by 

 the forceps and scattered over the base of the first antenna, whence some find their 

 way into the sac. A specimen which moulted and was confined in a basin of 



1 For the terms descriptive of the structures found in the eyes of Arthropoda see the general 

 description of that phylum. The Crayfish's eye would be described, using the terms there given, 

 as polymeniscous, diplostichons, retinulate, furnished with vitrellae, and perhaps also as exo- 

 chromic. But the origin of the pigment cells has not been exactly ascertained. See Lankester and 

 Bourne, Q. J. M. xxiii. 1883, on Eyes of Scorpio and Linmlus. 



