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Show 1893.] ORGANS OF LAMPREYS AND HAG* 731 viduals of Myxine glutinosa, and it is well known that Johannes Midler's substitution1 of the generic name Bdellostoma for Dumeril's " Heptatrema " was directly expressive of numerical variation of the branchiae In knowledge of these facts he (Prof. Howes) had lost no opportunity o2 seeking for additional evidence of variation of the parts in question, anu the condition of the heads n o w exhibited appeared to him sufficiently interesting to warrant the publication of his results. Petromyzon Jiuviatilis.-He exhibited two specimens (both " females " ) 2 . In the larger of them the first branchial aperture of the left side (fig. 1 a [which was visible only under a hand-lens]) was but one-third the size of its fellow0 and longitudinally instead of vertically disposed. Its free border was uniformly fimbriated and bounded, not an orifice, but a shallow cutaneous depression (br. I, Fig. 1 a. Petromyzon Jiuviatilis, 25'5 centim. total length, showing vestigial first gill on left side. Fig. 1 b. The same, dissected from beneath. Fig. 2 a. P. Jiuviatilis, 23'5 centim., showing absence externally of first gill on right side. Fig. 2 b. The same dissected from beneath. Fig. 3. Myxine glutinosa, dissection from beneath of a specimen possessed of a supernumerary gill on the left side. 1 a, 2 a, nat. size. 1 b, 2 b X 1£. 3 X 1£. Reference letters.-br., branchiae ; br.s., supernumerary brancbia; ce., peri-branchial blood-lympb sinuses ; d.ce., cesopbago-cutaneous duct; /., tegumental furrows; ph., pharynx laid open from beneatb; s., inter-branchial septum. fig. 1 b) which was closed internally. O n dissection, the ventral aorta was found to give off six branches on the left side and seven on the right; and, in accordance with this reduction, the gill corresponding to the vestigial orifice was entirely absent, its place being occupied by an extensive blood-lymph sinus (cc.) homologous with those of the normal peribranchial series. _ In the second specimen, the opposite (right) side was somewhat similarly affected in the corresponding region. Viewed externally (fig. 2 a) the 1 (" Myxinoiden ") Abbandl. Akad. Wiss. Berlin, 1834, p. 79. 2 Beard bas recently discovered tbe presence of ova in the testis of P.planeri, Brit Assoc. Rep., Edinburgh, 1892, p. 790, and Anat. Anz. vol. viii. p. 60, |
