1893.] 



OEGANS OF LAilPEEYS AND HAU. 



78i 



viduals of Myxine glutinosa, and it is well known that Johannes 

 Miiller's substitution ' of the generic name Bdellostoma for Dumeril's 

 "Heptatrema " was directly expressive of numerical variation of the 

 branchisB. In knowledge of these facts he (Prof. Howes) had lost 

 no opportunity of seeking for additional evidence of variation of 

 the parts in question, and the condition of the heads now exhibited 

 appeared to him sufficiently interesting to warrant the publication 

 of his results. 



Petromyzon fiuviatilis. — He exhibited two specimens (both 

 " females ") ^ 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-tliird the size of its fellows and longitudinally instead 

 of vertically disposed. Its free border was uniformly fimbriated and 

 bounded, not an orifice, but a shallow cutaneous depression {hr. I, 



jn ysh 



Fig. 1 a. Petromyzon fliiviatilis, 2b b ceutim. total length, showing Testigial 

 first gill on left side. Pig. 1 h. The same, dissected from beneath. Fig. 2 a. 

 P. fiuviatilis, 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 super- 

 numerary gill on the left side. 



\a,2a, nat. size. 1 6, 2 6 X li- 3 X li. 



Reference letters. — hr., branchias ; hr.s., supernumerary branchia ; cc, peri- 

 branchial blood-lymph sinuses ; d.oe., cesophago-cutaneous duct; /., 

 tegumental furrows ; ph., pharjTix laid open from beneath ; s., inter- 

 branchial septum. 



fig. 1 h) which was closed internally. On dissection, the ventral 

 aorta was found to give ofi six branches on the left side and seven 

 on the right; and, in accordance with this reduction, the gOl 

 corresponSng 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 ("Mj-xinoiden") Abhandl. Atad. Wiss. Berlin, 1834, p. 79. 

 ^ Beard has recently discovered the presence of ova in the testis oiP.planeri, 

 Brit. Assoc. Eep., Edinburgh, 1892, p. 790, and Anat. Anz. vol. viii. p. 60. 



