ON THE GENERAL MORPHOLOGY OF THE MYXINOID FISHES. 751 



most posterior section of the obliquus, the first of which passes external to the first 

 post-cloacal slime sack (instead of internal to it, as in the pre-cloacal slime sacks), to meet 

 at a mid-ventral linea over its anterior half. This course is the less surprising when 

 we remember that in Bdellostoma, according to J. Muller, the obliquus always passes 

 external to the slime sacks. In addition, however, to the above, the first pair of post- 

 cloacal slime sacks have a complete set of constrictor fibres belonging to the transversus 

 caudalis. 



We may now dissect out the two rows of post-cloacal slime sacks intact, together 

 with the transversus caudalis, and examine the muscle in situ. All the slime sacks 

 except the first pair, which are symmetrical, and the second pair, which are somewhat 

 asymmetrical,* alternate ; and not only is this the case, but space is so limited that they 

 actually interdigitate, and hence all the external apertures except the first pair strictly 

 alternate. The two rows are bound together into one coherent whole by the transversus 

 caudalis, which can thus act as a powerful constrictor to them. The muscle arises at 

 an irregular, ill-defined dorsal linea, which zigzags from side to side, following the 

 depression between the dovetailed slime sacks. Over the first two pairs of slime sacks, 

 however, this linea is straight, and lies in the mid-dorsal position. From the dorsal 

 linea the fibres generally pass more or less at right angles to the long axis round the 

 lateral surfaces of the slime sacks, which (except the last three or four) are completely 

 covered by them dorsally. The fibres continue their course round the sacks, and the 

 two sides become continuous on the ventral surface, i.e. there is no ventral linea. They 

 do not generally diverge to pass round the openings of the sacks, but many of them 

 end on the sack itself just above the opening. Hence there are fewer fibres below the 

 apertures than above them, although in places fresh fibres are formed below. The 

 muscle is usually specially strong at the depression between the sacks, but, except in 

 the case of the first pair, the fibres encircle both rows of sacks, and do not extend over 

 the opposed surfaces, i.e. between them ; and in fact the sacks are so closely packed 

 together that there is no room. Hence there is only one constrictor for both rows, 

 which must therefore be discharged simultaneously without the power of independent 

 action. At the mid-ventral line the fibres diverge to allow the exit of the ventral 

 "fin rays." Posteriorly some of the fibres, as they traverse the lateral surfaces of the 

 sacks, pass forwards, whilst others course backwards, so that they cross at an angle. 



36. M. cordis caudalis. (Figs. 4, 7, cd. c.) 



G. Retzius, 15, p. 96. 

 Greene, 8, p. 367. 



A curious small fan-shaped paired muscle, largely covered by the parietalis, the 

 handle of the fan or insertion pointing straight forwards. It arises as follows : — (a) the 

 dorsal fibres have a long origin from the skeletogenous layer of the notochord at the 



* This is subject to variation. 



