The _system of sensory canals_ consists of the following members on each side of the head:— A _supra-orbital_, passing above the eye to the end of the snout and bending back on the ventral surface to join the infra-orbital. An _infra-orbital_, which branches off from the supra-orbital and passes ventrad between the eye and the spiracle, then turns forward along the ventral margin of the orbit, and finally bends toward the midline and extends to the tip of the snout. A _hyomandibular_, which leaves the infra-orbital below the eye and runs back beyond the angle of the mouth. A short _mandibular_, on the mandible close to the angle of the mouth, which is not connected with the other canals. The canal systems of the right and left sides are connected by a _supratemporal_ canal just behind the endolymphatic pores, and often by an anastomosis of the infra-orbital canals in front of the mouth. On some specimens two crescentic rows of pores can be found between the bases of the pectoral fins, which represent a third type of sensory organ, the _pit organs_, closely related genetically to the sensory canals. Two longer lines of pit organs, (the _mandibular pit organs_), will be found a short distance behind the mouth. Similar pit organs are found in front of the endolymphatic pores, and above the anterior part of the lateral line. Except for a few small areas the entire surface of the body is covered with small, sharp-pointed denticles (_placoid scales_). Each consists of a diamond-shaped basal plate embedded in the dermis, from which projects a leaf-like, backward directed spine. A piece of skin should be removed and examined under a low magnification. The dermis is so dense and pigmented that the basal plate is not easily studied without further manipulation. For this purpose boil a piece of skin in 5% caustic potash solution until it is softened, but not till it disintegrates. Then clear it in glycerine. Examine the individual denticles under a higher magnification. The denticle consists of dentine, the spine being of a much denser structure than the base. The teeth and the large spines of the fins and claspers are also composed of dentine and may be considered as modified placoid scales. Denticles, teeth, and spines are covered with a shiny, enamel-like layer which, however, does not appear to be true enamel such as covers the teeth of higher vertebrates. The shape of the scales and their closeness vary on different regions of the body, and there are certain regions entirely free from them, namely, back of the dorsal, pectoral and pelvic fins, the medio-dorsal surfaces of the claspers, inside the upper lip and the labial pockets. DISSECTION OF THE ABDOMINAL VISCERA