" POEUS GEKITALIS " IN THE MYXINID^. 493 



some o£ these are in reality spider-cells I cannot definitely say, 

 but thread-cells can be certainly seen here and there buried in 

 the tissues. 



From the foregoing descriptions, the great difference that 

 exists between the Myxinoids and their nearest relatives — the 

 Lampreys — with regard to their " pori genitales " has, I trust, 

 been made sufBciently clear. The explanation o£ the difference 

 is most likely bound up with the function performed by the 

 pores, for in the Marsipobranchii their chief duty is important 

 enough to account for any suitable modification of their structure, 

 since they form the sole passage by which the generative products 

 can reach the exterior. The " genital pores " in the Lamprey, 

 in which the reproductive elements are small (the ova being 

 about the size of a small shot), are, as we have seen, paired and 

 of such size ("5 mm. in P. fluviatilis *) that, under pressure of 

 the ripe ova in the body-cavity, they expand enough to allow of 

 their passage t. On the other hand, the Myxinoids possess ova 

 few in number but large in size (a fairly ripe ovum of Myxine 

 being a spindle-shaped body 19 mm. in length by 7 mm. in 

 diameter, and that oi Bdellostoma 31 mm. in length and 9'5 mm. 

 in diameter). Eor the passage of such an ovum, pores similar 

 to those of the E-iver Lamprey would be clearly too small and 

 certain to suff'er extensive rupture if the ovum succeeded in 

 making its exit. But instead of such pores, we find in Myxine 

 a single median pore of very considerable size, and in Bdellostoma 

 an enormous pore divided into two, in both cases admirably suited 

 for the passage of a large-sized egg. For not only is the aperture 

 large, but, to guard against any possible danger of rupture, it is 

 encircled by a stout band of connective tissue, uncalled for in 

 the case of the Lamprey with smaller eggs. What the special 

 use of the slime secreted round the edge of the pore in Myxine, 

 and more copiously in the anterior cloacal prolongation of Bdel- 

 lostoma, may be, it is not very easy to say. It may of course have 

 a certain lubricating effect in oviposition; but to judge from the 



* These remarks do not apply to the Sea Lamprey, in which the pores are 

 larger than the size of the egg would appear to wai'rant. Ewart speaks of their 

 diameter as nearly 2 lines in the species examined by him (P. marinus), and 

 in a specimen in the Royal College of Surgeons Museum I find them fully that 

 size. 



t GuUiver (Proc. Zool. Soc. 1870, p. 848) found 51,220 ovarian eggs in 

 P.fluviaiilis, and gives jV in. as their diameter ; in P. Planeri the diameter was 

 about gJjj in. 



