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Show 1889.] ANATOMY OF GONATUS FABRICII. 121 upper surface is curved so as to fit into the under surface of the cartilaginous pad which lies below the pen. A little further back (Plate XIV. fig. 1) two processes are seen, one on each side, passing downwards into the muscles, which thus take their origin from them. Still further backwards these processes become longer and pass completely through the body-wall, separating the muscles, which here compose it, into quite distinct masses. The outer limb of the collaris muscle (co".) springs from the notch between the horizontal plate of the cartilage and its vertical process, whilst the inner limb (co'.) is attached to the internal surface of the process. As this last lengthens, however, the muscular attachment shifts from the inner surface to the narrow terminal edge of the vertical process. Three small cartilages which lie below and on either side of the pen will be considered subsequently. IV. The Pen. The larger specimens presented nothing more than fragments of the stem of the pen, but in the smaller ones transverse sections which were made through the anterior part of the mantle and two series of longitudinal sections through its posterior extremity yielded some information regarding the structure and relations of the pen and pen-sac; and therefore since, so far as I am aware, no observations upon these organs in the Decapoda have hitherto been published, I propose to devote a few words to their description. A transverse section made about a millimetre behind the margin of the mantle is figured on Plate XIV. fig. 1. The pen itself (p.) has the figure of an arch with everted limbs, which lies on the whole a little nearer the ventral than the dorsal surface of the mantle. In most instances the process of cutting has produced cracks in it, which indicate its composition of layers parallel to its upper and lower surfaces. It is covered on both sides by an epithelial layer. In the preparations the contraction of the tissues has drawn the lower layer away from the pen, but there can be no reasonable doubt that during life they were in apposition. The upper layer of epithelium (e) consists of very small flattened cells, shortly oval in section and with nuclei of corresponding form ; they contain only a small amount of protoplasm. The epithelium below the pen (e ), on the other hand, is immensely thicker, consisting of a single layer of elongated columnar cells. The protoplasm of these cells stains more deeply in the distal than in the proximal portion, and is very faintly granular. The nucleus is situated about one third from the proximal end of the cell, and contains a deeply stained nucleolus at its distal end. The concavity of the arch is partially occupied by a pad of cartilage (s.nu.), which is thickest in the middle line and thins away, at first suddenly, then more gradually towards the extremities. The form is such that this cartilage fits exactly into the other one which is embedded in the muscles in the dorsal median line. This is not apparent in the figures because these were drawn from different sections.. The two concavities which are due to the eversion of the limbs of the arch are also filled by pads of cartilage (su.g.) similar |