ON THE APPENDICES GENITADES (CLASPERS) IN THE SELACHIANS. 23 



such a degree that it does not appear as an independent muscle, and only artificially is to be separ- 

 ated from the former. 



The distal part, the muscular system of the shaft, is typically composed of two muscles: M. 

 dilatator (/)), and M. compressor (saccz) (S). 



M. dilatator [/>) is always very large and powerful; it wraps in a cloak-like manner the appen- 

 dix-stem until the terminal part, leaving only the lateral surface uncovered, part of which is occupied 

 by M. compressor. M. dilatator originates forward, either from the appendix-stem only, or frequently 

 also, above the knee of this latter, from the pieces 6 l} etc., or from the basale; posteriorly it is 

 attached to the aponeurotic wrapping of the terminal part, or, when covering pieces have been devel- 

 oped from the wrapping, partly to these. Besides fibres of it often go to the skin, and here and there 

 bundles pass into the M. compressor. The chief action of this muscle is to bend the terminal pieces 

 together with the soft end-style (veutro-) medially, by which means the terminal part of the appendix- 

 slit is widened; at the same time some of the terminal pieces are often turned from their position 

 of rest in such a manner that they rise through the skin, or are erected so that they stand out free 

 (as the spur or thorn in Sonmiosiis and Lauiua, the -claws in Spinax, the hook and the spur in 

 Acanthias; the large piece T^ in Ra/a, etc.). When the contraction ceases the appendix-slit will again 

 be narrowed, and the erected skeletal pieces will again be laid, partly mechanically by elastic 

 reaction of the soft connective tissue, but partly also the M. compressor will be able to support this 

 latter operation. 



Muse, compressor shows in the Plagiostomes so particular a structure, that when it has been 

 examined at all, it has hitherto been misapprehended, the greater part of it being understood as a 

 bag composed of dermal muscles. 



This muscle, I suppose, originally occupies in the Plagiostomes a place, similar to that in the 

 Holocephales (see later); i.e. it covers the lateral surface of the appendix-stem, or very frequently 

 only its proximal part, and anteriorly it also reaches on to the piece t 3 and the (two) last rays. Into 

 this muscle, a longitudinal folding of the outer skin penetrates from the dorsal side of the shaft; this 

 folding forms the appendix-slit and the glandular bag, the former leading into the latter. The fore- 

 most part of the folding growing on ventrally, carries with it the wrapping muscle, and then both 

 grow on together, and form a singularly thickwalled bag which from the slit-formed opening on the 

 dorsal side grows 011 between the last ray and the stem skeleton to the ventral side of the fin, where 

 it becomes situated between the outer skin and the ventral ray-muscles. In Sharks the foremost, 

 blind part very often grows much farther forward, not only near to the pelvis, (Spinax, Rhiua, 

 Somniosas) but in many, I think in most Sharks it reaches forward of the pelvis (for inst. Acanthias, 

 Scyllium, Pristiurus, Laiiuia, Selae//us)'), and then the bags of the two sides are in contact a long 

 way in the median line (fig. 4). In the Rays the bag is much smaller, (pi. VI, fig. 68), but on the other 



M In Mustehis /avis the glandular bag reaches as far forward as to the pectorals; i. e. the part before the pelvis is 

 of far more the double length of that behind it. I have myself only had immature males of Must, tavis for examination ; 

 but this statement I found on a drawing without any text, left by A. Schneider which, together with other drawings, has 

 been published as an appendix to the fragment left by S. : Studien zur Systematik und zur vergl. Auat., Eutwickelungsge- 

 schichte und Histologic der Wirbelthiere 1Z00I. Beitrage, vol. 2, 1890). The figure in question (pi. 25, fig. 11) is explained as: 

 Mustelus l&vis. Brustflossen und Bauchflossen mit Saamenblasen. Die Cutis entfernt. Bauchseiu-. 



