ON THE GENERAL MORPHOLOGY OF THE MYXINOID FISHES. 785 
many more fin rays in Myxine than in Bdellostoma—as, for example, in the specimen 
figured there were altogether 147 fin rays in the caudal fin as against a maximum of 
ninety-four in Bdellostoma, according to AyERS and JAcKsoN. 
Median Ventral Bar ( m. v. b.).—Commences as a short thin rod closely attached 
to the mid-ventral region of the notochord. In front, one or two nests of cartilage cells 
may occur in the same position (fig. 17). This rod soon widens out, by a concave 
forward sweep, into a wide plate (with an expanded chordal base) to which the ventral 
fin rays are fused. In Bdellostoma, according to AvERs and Jackson, the bar arises far 
forwards as a pair of long slender rods which fuse behind in the mid-ventral line. 
This is, therefore, very different from Myxine. The bar sends forwards a prominent 
projection with a rounded extremity which gives origin to the pair of wide, somewhat 
diffuse, muscle sheets of small fibres which pass one on each side of the caudal hearts 
externally, as described by G. Retzius, and which are responsible for the pulsation 
of these organs. In one specimen there was a perforation in this bar near the anterior 
end which [| have since failed to find either by dissection or in serial sections, and which 
transmitted an anastomosis between the caudal hearts. As the median ventral bar 
passes backwards, and the.notochord is tapering down, its base becomes more expanded 
and begins to creep up at the sides of the chorda. At the same time, a small rod of 
cartilage is deposited dorsally on each side in the angle formed by the base of the fibrous 
neural tube (sp. c.) and the roof of the notochord, one or more nests of cartilage cells 
being found at short intervals in front of these rods (two are shown in fig. 17). The 
rods and the median ventral bar then suddenly fuse, so that the chorda is now completely 
invested with cartilage, except dorsally. Shortly afterwards this compound rises up and 
fuses with the median dorsal bar, thus forming a complete cartilaginous neural tube 
except that its floor is formed by the roof of the chorda. The latter itself becomes here 
eradually invaded by cartilage cells and is soon almost entirely merged into the median 
ventral bar, with the result that the cartilaginous neural tube is finally completed at its 
only lacuna—-the base. However, nests of notochordal tissue occur at intervals 
embedded in the cartilage behind this region, thus indicating that the chorda extended 
further back than appears in the adult. Immediately behind the termination of the 
chorda a large fenestra arises on each side of the tube formed by the fused median 
dorsal and ventral bars, and the spinal canal is thus exposed laterally. The latter, 
however, does not entirely fill this fenestra, even at its expanded termination lodging 
the curious dilated filum terminale,* as shown in the figure. The ventral edge of the 
bar bears thirty-three fused fin rays, of which only one was bifurcated in the specimen 
figured, as against the five bifurcating rays attached to the dorsal bar. G. Rerztus gives 
about thirty, but does not figure any bifurcating rays although such are mentioned in 
the text. In bdellostoma, according to AvERs and Jackson, there are only about twenty 
fused ventral rays, of which nearly all are bifurcated, and the median ventral bar 
extends forwards under the notochord as far as the cloaca without fusing with the 
* In Bdellostoma, according to Ayers and Jackson, the spinal cord is not dilated. 
