466 MERISTIC VARIATION. [part I. 



the large numbers of birds, wild and domesticated, annually dissected 

 in laboratories it may perhaps be concluded that these variations are 

 exceedingly rare 1 . 



723. Hen having a small right ovary in addition to the left ovary. The 

 left oviduct was normal, but the left ovary was partially transformed 

 into sacculated tissue. [Full histological details of the structure of 

 both ovaries given.] The hen had partly assumed the plumage of the 

 cock, having four sickle-feathers and other characters proper to the 

 male. Brandt, Z.f. w. Z., xlviii. 1889, p. 134, Pis. 



724. Hen having a normal left oviduct and in addition a partially 

 developed right oviduct which formed a large thin-walled cyst dis- 

 tended with gas. C. S. M., Ter. Cat, 1872, 455. 



Proboscis-jiore of Balanoglossus 

 and water-pores of larvae of Asterias. 



*725. Balanoglossus kowalevskii. The anterior or proboscis-body- 

 cavity is continued backwards into the proboscis-stalk as two hollow 

 horns. In this and most other species the left of these alone acquires 

 an opening to the exterior at the proboscis-pore. In B. kupfferi alone 

 there are two such pores, one opening into each of the two horns 2 . A 

 specimen of B. kowalevskii in which both horns thus opened to the 

 exterior was seen by Morgan, T. H., Jour, of Morph., 1891, v. p. 442. 

 726. Asterias vulgaris. The Bipinnaria larva as commonly seen 

 resembles the usual Tomaria in having a left water-pore only. In 

 several larva? 3| to 4 days old the presence of two such water-pores, a 

 right and a left, symmetrically placed, has been observed by Field and 

 Brooks. The right pore subsequently closes. This condition is be- 

 lieved by Field to represent not a variation but a normal phase of 

 development [though further confirmation is needed]. Field, G. W., 

 Q. J. M.S., 1893, xxxiv. p. 110, PI. xiv. figs. 22 and 23. 



Variations in Flat-fishes. 



A curious series of variations bearing on the relations of the 

 right side to the left occur in Pleuronectida3. The evidence on 

 this subject was collected by Steenstrup 3 in 1863. 



Flat-fishes are normally coloured on the upper side and are 

 without chromatophores in the skin of the lower side 4 . Variations 

 in colour occur in two ways ; the upper side may be white like the 

 lower, or on the contrary the lower side may be coloured like the 

 upper. The former change cannot well be distinguished from 

 other cases of albinism 5 and does not call for special notice here. 



1 In view of the cases of the Crayfish and the Cockroach mentioned in the 

 Preface, much stress cannot be laid on this consideration. 



2 Spengel, J. W., Mitth. zool. Stat. Neap., 1884, v. p. 494, PI. xxx. fig. 2. 



3 Steenstrup, Overs, k. Dansk. vid. Selsk., 1863, p. 145, abstr. by Wyville 

 Thomson, Ann. and Map. N. H., 1865 (1), p. 361. 



4 In some species the coloured side is normally the right, in others the left, 

 reversed specimens being common in some species (P. flesus), rare in others. The 

 reversed condition concerns only the head, skin, muscles, &c, and there is no 

 transposition of the internal viscera. 



Evidence collected by Steenstrup. Gottsche (Arch. f. Naturg., 1835, u. p. 

 139) states that P. platessa is not rarely wholly white on both sides. I have never 



