244 TRANSACTIONS OF THE [JUNE 5 



there is a loose fold of the integument continuovis behind with 

 the median raphe or keel of the tail. The cloacal aperture 

 occurs at the end of a small papilla, which is often completely 

 hidden beneath this loose fold on the right side of it. The 

 cloacal papilla, however, undergoes varying degrees of con- 

 crescence with the fold and, accordingly, sometimes the aper- 

 ture lies quite near the extremity of the latter, but still some- 

 what to the right of the middle line, as in Fig. 2. 



In one case only, out of the many that I examined, was the 

 cloacal aperture placed on the mid-ventral line, as in Fig. 3. 



In another isolated instance, a very striking condition was 

 observed (see Fig. 1). The hinder end of the above-mentioned 

 loose integumentary fold was bifid, and presented at first sight, 

 a perfectly symmetrical appearance. On close inspection, how- 

 ever, it became evident that the left half of the fold was con- 

 tinuous with what has been referred to above as the raphe or 

 keel of the tail, while the right half hung freely and bore the 

 cloacal aperture at its tip. 



I am not aware that attention has ever been drawn to this 

 asymmetrical position of the cloacal aperture in the Batrachian 

 tadpole, nor to what extent it occurs in other species. It is 

 certainly not universal. In the tadpoles of Bnfo lentiginosus 

 for instance, the external opening of the cloaca lies normally in 

 the mid-ventral line, whereas, we have seen that in those of E. 

 elamaia the asymmetrical position is the usual one, and the 

 median position is the exception. 



Heron-Royer and Ch. van Bambeke, in their researches on 

 " Le vestibule de la bouche chez les tetards des batraciens 

 anoures d'Europe," (Archives de Biologic IX., 1889, pp. 185-309, 

 PI. XII. -XXIV.), give diagnoses of the different tadpoles 

 examined by them, but make no mention of any peculiarity in 

 the position of the cloacal orifice. 



It is interesting to call to mind the well-known fact, that in 

 the lung-breathing fish, Protopterui^, there is an analogous asym- 

 metry in the position of the cloacal aperture, lying as it does, 

 now to the right and now to the left of the median line, (cf. 

 Wiedersheim, Lehrbuch d. vergl. Anat. der Wirbelthiere, 2 

 Auflage, 1886, p. 563). In the specimen of Prolopleru^ annectens 

 in the adjoining Geological Museum, it lies to the leff of the 

 middle line, being placed at the side of a mesial fold of the skin, 

 which is of precisely the same nature as that described above for 

 the tadpole of B. clamata. 



It also recalls the condition of things in Amphioxus, where the 

 anus lies constantly on the left side of the bod}', above the 

 caudal fin. 



