The optocoels are large cavities within the optic lobes which open into the aqueduct of Sylvius. A large metacoel in the metencephalon opens into the myelocoel. The myelocoel is also continued into the corpora restiforma; posteriorly it joins the _central canal_ which extends down the center of the spinal cord. DISSECTION OF THE EYE. Remove one of the eyes from its orbit, and divide it into inner and outer halves by an equatorial cut around the eyeball (not directly through it, as this tears the lens from its fastenings). Place the halves under water and observe: In the inner half: The _posterior chamber_, the cavity of the eyeball which has been opened. During life it is filled by a gelatinous substance, the _vitreous humor_. The _retina_, a delicate yellowish-white membrane lining the interior of the eye, loosely attached to the outer coats except at the point of entrance of the optic nerve. The _choroid coat_, a thin, black membrane outside the retina. It can be pulled away from the outer coat quite easily except near the optic nerve. The _sclerotic coat_, the outer coat of the eye. This is composed of connective tissue having an almost cartilaginous consistency, is only slightly pigmented, and is somewhat translucent. The muscles of the eye are inserted upon the sclerotic. In the outer half: The _ora serrata_, an irregular line along which the retina ends. The _iris_, a fold of the choroid extending inward like a shelf, and perforated centrally to form the pupil. Around the iris the choroid is folded radially into the _ciliary processes_. The _lens_, a spherical body, transparent and elastic during life, but opaque and hard in preserved specimens. It projects into the pupil and is suspended from the ciliary processes by a delicate membrane, the _suspensory ligament_. The _anterior chamber_, in front of the iris and lens, filled with a watery fluid, the _aqueous humor_. The transparent _cornea_, forming the outer side of the eyeball, continuous with the sclerotic. Take out the other eye and cut it in two by a section through the pupil and optic nerve. Review the relation of the parts. THE SKELETON There seems to be no easy way of cleaning the skeleton of dogfish which have been preserved in formalin or alcohol, the only procedure being to cut, pick, and scrape the flesh away from the skeleton. Time and patience are required, but if these are allowed there is no reason why all the parts of the skeleton cannot be thoroughly studied. Specimens which have been preserved in brine are more easily skeletonized. The skeleton is entirely composed of cartilage which, in large species of elasmobranchs and in old individuals of small species, becomes impregnated with lime salts, in some cases to such an extent as to resemble soft bone.