REPORT ON THE MOLLUSC A. 31 



1. In several forms one may observe an exaggeration of tlie first exception cited 

 above, that is to say, rudimentary condition, in consequence of the conceahnent of the 

 eyes under organs which completely cover them. All such organisms are burrowers, 

 like the Naticidse and Bulloidea referred to above. 



A. Diphyllidia} 



B. Several species of Terebra.^ 



C. Certain Olividse : Agaronia,^ Olivella,* AnciUanaJ 



D. Bulla.' 



2. It is well known that the eyes of pelagic animals have a tendency to become 

 very perfectly developed, or, on the contrary, to become rudimentary, and to disappear. 

 As an example of the former specialisation, one might cite Heteropods among Molluscs, 

 Alciope among Annelids, &c. As to the second direction, we have already seen the 

 tendency to atrophy exhibited by Gastropods, e.g. in Phyllirhoe cited above. But the 

 tendency to become rudimentary may become still more marked. 



A. In certain pelagic Nudibranchs, such as Glaucus (where the eye is situated on 

 the central nervous system, and has become quite microscopic)." 



B. Among " Pteropods," several forms (such as Pneumonoderma, Clione) exhibit the 

 rudiment of an eye which does not appear to be any longer functional. Certain forms 

 of Clio (Creseis) still possess two pigmented spots, bearing several minute refractive 

 bodies. The other forms no longer exhibit any trace of an organ of vision. 



C. In Janthina, the older authorities. Lesson, Eang, d'Orbigny, assert the presence 

 of eyes, but they are not agreed even in regard to the position of these organs. On the 

 other hand, all the other authorities, Quoy et Gaimard, Delle Chiaje,' Clark," the 

 brothers Adams," Gwyn Jeffreys,'" von Jhering," Bouvier,'' and myself, are agreed on 

 this point, that no organs of vision were to be found in any of the different forms 

 examined. 



^ Siebold, loc. cit., p. 316 ; Souleyet, he. cit., Mollusques, pi. xxiv. E, figs. 16, 17. 



-Woodward, A Manual of the Mollusca (1856), p. Ill; Broiin (Keferstein), Die Klassen und Ordnimgen des 

 Thierreichs, Bd. iii. p. 1046 ; Bouvier, Systeme nerveux, morphologie gencrale et classification des Gasteropodes 

 I'rosobratiches, Ann. Sci. Nat. (Zool.), s^r. 7, t. iii. p. 322. 



•■' Woodward, loc. cit., p. 117. * Fiscber, Manuel de Conchyliologie, p. 599. 



^ A. and H. Adams, The Genera of Recent Mollusca, t. i. p. 112. 



" Bergh, Anatomiske Bidrag til Kundskab om Aeoliderne, A'. Daimk. Yidenak: StUk. Skrir., t. vii. (1864) 

 p. 265; Yayssiere, Observations sur I'anatomie du Glaucus, Ann. Sci. Nat. (Zool.), scr. 6, t. i. p. 15, pi. x. 

 fig. 6, s. 



' Descrizione e notomia degli Animali senza vertebre, pi. 67, 68. 



" On the Janthinse, Scalarise, Naticae, Lamellarise, and Velutinte, Ann. Mciij. A^at. Hist., ser. 2, vol. xi. p. 48. 



' The Genera of Recent Mollusca, t. ii. p. 85. '" British Conchology, vol. iv. p. 82. 



" Vergleichende Anatomie des Nervensystemes und Phylogenie dcr Mollusken, p. 108. 



'- Contributions ii I'ctude des Prosobranches Ptenoglosses, Bull. Soc. Mulacvl. France, 1886, p. 81. 



