NUD I BRANCH I ATA. 19 



Dorididce they form a circle on the central line, and in other 

 families they are disposed in rows along the back or sides ; in 

 one section or snborder (Pellibranchiata) the entire surface 

 or skin is the sole respiratory organ. Sexes united in each 

 individual, which is strictly hermaphrodite, but with distinct 

 organs of generation. 



SHELL present only in the fry or embryonic state, and fur- 

 nished with an operculum ; it is rudimentary and resembles 

 in shape a miniature Nautilus or Helix, having scarcely more 

 than a single spire, which is reversed or sinistrorsal, as in the 

 larval shell of Aplysia. 



Until the present century all these mollusca were 

 placed in two or three genera and consisted of about 

 20 species. Now the British Nudibranchs alone repre- 

 sent 12 families, 25 genera, and 111 species. Linne, 

 Forskal, and Miiller confined their observations to the 

 external form. Bohadsch paved the way to a more 

 complete investigation, which was systematically carried 

 out by Cuvier in his admirable 'Memoires/ During 

 this century a succession of other zoologists, including 

 Rapp, Ehrenberg, Risso, D'Orbigny, De Blainville, 

 Milne-Edwards, Delle Chiaje, Couthouy, Leuckart, 

 Quoy and Gaimard, Philippi, Cantraine, Verany, Sars, 

 Loven, Cavolini, Quatrefages, Nordmann, Alexander 

 Stuart, especially Bergh, Meyer and Mobius, and, 

 among our own countrymen, Montagu, Fleming, Forbes, 

 Johnston, Allman, Reid, Leach, Dalyell, Collingwood, 

 and Mlntosh, have greatly extended our knowledge of 

 this peculiar group. But above all these must be ranked 

 Mr. Joshua Alder and Mr. Albany Hancock, the authors 

 of the most complete monograph that has yet appeared 

 in the history of the Mollusca. It is one of the many 

 valuable works of the Ray Society, and took ten years 

 (1845-1855) in the publication. This monograph is 

 so excellent, that I cannot do better than give their 



