OCR Text |
Show 120 MR. W. E. HOYLE ON THE [Mar. 5, preserved : in one the cartilage was sacrificed to the dissection nervous system, and in the other it had already suffered a good deal of mutilation. I made a sketch of the fragments put together as carefully as possible, and afterwards compared this with the series of sections through the head of one of the smaller specimens. The two figures (Plate XIII. fig. 6) do not therefore represent a drawing of an actual specimen, but have been put together from these two sources. The basal portion of the cartilage, situated below and behind the united pedal and visceral ganglia, consists of an oblong box with a longitudinal septum down its middle. Each division is produced backwards as a blunt prominence ; these are the receptacles in which the auditory organs are lodged. Anteriorly the sides of this box converge and the lower portion of the cartilage comes to consist of a vertical keel, which splits at its lower margin into two plates passing outwards and downwards below the eyeballs. These plates are each perforated near their outer margin by a foramen (osp.), which serves to transmit the nerve to the osphradium. At the point where the sloping subocular plate joins the vertical keel is a large foramen (vf".), which serves for the passage of the veins from the eyes into the large vena cava. Another foramen in the middle line (vf.) gives passage to the veins from the central nervous system. From either side of the box a vertical plate is given off which lies upon the posterior surface of each eye. The vertical sides of the box are produced upwards and support a horseshoe-shaped plate, the concavity of which is directed backwards. The convexity extends much further forwards than the vertical supporting plates, and arches over a space in which the cerebral ganglia are situated ; its anterior extremity is bluntly pointed. There are no basi-brachial cartilages such as are seen in Sepia, but there is a thin plate of that material in the dorsal wall of the vena cava behind the cephalic cartilage and quite disconnected from it. The nuchal and siphono-articular cartilages (Plate XIII. fig. 7) present no special characters worthy of note. The latter are of the linear kind common to the greater number of Decapoda ; they exhibit a tendency to a slight curve, in the form of f. Of more interest is the presence of a pair of cartilages in the ventral wall of the siphon on its outer aspect. These are two thin plates (Plate XIII. fig. 7) of the form roughly of an isosceles obtuse-angled triangle, the median border being straight, the outer expanding to an angle. The sheet of cartilage is thickest near the middle Hue and thins out gradually towards the side, losing itself in the surrounding tissues. Their formation had not commenced in the smaller specimens; compare Plate XIII. fig. 5. The basi-pterygial cartilages had, however, developed to a considerable extent in these examples. The extremity of the pen lies in a groove between them. The pallial cartilages have a much more intimate connection with the structure of the body-wall than seems to me to have been hitherto recognized. The nuchal cartilage commences as a thin plate which lies upon the muscles in the dorsal median line. Its |