444 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



the ear, appears to arise as au outgrowth from the autennular fiber mass of the brain (Fig. 243, 

 af.). It consists of a fibrous portion leading directly from the fiber mass of the brain and of 

 soniewliat flattened, or spindle-shaped nuclei, which penetrate the cortex of nervous cells and are 

 undoubtedly proliferated from them. Possibly the internal sheath is continued over the growing 

 nerve. 



IX. —THE EYES. 



The eyes in Decapods consist of a pair of lateral compound eyes, which in the larva are 

 mounted upon long stalks, a condition usually retained in the adult, and of a median ocellus. In 

 the Alpheus family the compound eyes are nearly sessile and are completely hooded by the cara- 

 pace. In the larva, however, the eyes are both naked and possess long, movable peduncles (see tlie 

 inetaiuorphosis, PI. xxi). In Alpheus saidcyi the oi)tic stalks are reduced to a mere rudiment, and, 

 though provided with muscles, they can possess but very slight power of movement. They meet 

 on the middle line in front of the brain, over the roots of the first pair of antennse. The compound 

 eyes face forwards and outwards at an angle of 45° with the mesial plane of the body. A median 

 papilla projects from below the lateral eyes, bearing the pigmented ocellus (Figs. 20!», 210, of 

 larva). Two small hairy tubercles, outgrowths of the integument, occur on the inner side of the 

 optic stalk. 



Spence Bate (3) says that since the Alphei are frequently found in ooze and muddy bottoms, he 

 is inclined to think that they burrow more extensively than the common shrimp, and that the 

 carapace has become modified to protect the eyes on this account. This may be true, but the 

 Squilhe, on the other hand, in which the burrowing habit is more characteristic, have undergone 

 no such modification. 



THE MEDIAX EYE. 



The median " nauplius" eye persists in the adult Alpheus, and is probably functional to some 

 extent as a visual organ. This is represented in Fig. 18, PI. xxii, where it is seen as a small 

 conical papilla, lying between the basal joints of the antennnles, and at the roots of the compound 

 eyes, very close upon the brain. This is the first instance I have noticed of the persistence of the 

 nau])lius eye in the adult. 



The general position of the median eye of the first larva of Alpheus saulcyi is seen in Figs. 

 209 and 210, and the minute structure in Fig. 197. The pigment takes the form of an inverted 

 Greek capital upsilon (j). There is no lens. A coaguable fluid is present, which is probably 

 blood plasma. Besides muscle-fibers and pigment-secreting cells, and the integumentary epithe- 

 lium, these ectodermic nerve end (?) cells, which abound toward the center of the papilla, are 

 continous backward into the cortical cells of the anterior fiber mass of the brain. 



The first trace of the frontal eye which I have noticed occurs in the tenth stage (Fig. 168), 

 when a small number of cells developing black pigment (probably ectodermic) can be seen near 

 the surface, on the middle line, next the anterior extremity of the brain,* 



GENERAL ANATOMY OP THE EYESTALK. 



The visual apparatus of the compound eye of a Crustacean consists ef three principal parts, 

 the retina, the optic ganglion, and the optic nerve, uniting retiua with ganglion, in addition to a 

 peduncle of nerve fibers, which jiuts the optic ganglion in communication with the brain. 'These 

 parts are contained in the eyestalk or opthalmite. The eyestalks are covered with a cuticle 

 secreted by the ectoderm like that over the rest of the body. This cuticle, which in some prawns 

 like Steuopus is hard and armed with spines, is converted at the distal, hemispherical surface of 

 the stalk into a transparent cornea. The ectoderm or hypodermis secretes a well defined basement 

 membrane. This is continuous with the basement membrane of the retina, pointing clearly to the 

 fact which development proves, that the retina is a differentiated portion of the hypodermis. 

 Below the basal membrane we meet with blood vessels or sinuses, muscles which control the 

 movements of the eyestalk, glands, connective tissue, ganglion cells, and fibrous substance. 



* While tliis iiipmoir was in press a short paper appeared in the (}uarlerhi Journal of Microscopical Science (January, 

 189'2), " 0« tlie XaupHus Eye persislimj in some Dicapotls." hj Jtlarf^aret Robinson. Tbe median eye was oliserved in 

 some eight different species of the Carididie, including the genera Palajuion, Hippolyte, Virbius, Crangon, and Pan- 

 da! us. 



