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 com- 

 pletely through the body-vvall, 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, 

 1 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 hes 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 com- 

 position of layers parallel to its upper and lower surfaces. It is covered 

 on both sides by an epithelial layer. In the preparations the con- 

 traction of the tissues has drawn the lower layer away from the pen, 

 but there can be no reasonable doubt that during hfe they were in 

 apposition. The upper layer of epithelium (e) consists of very 

 small flattened cells, shortly oval in section and with nuclei of corre- 

 sponding 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 con- 

 tains a deeply stained nucleolus at its distal end. 



The concavity of the arch is partially occupied by a pad of car- 

 tilage {s.mi.), 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 



