128 SCIENCE PROGRESS 



function of the shell, until the latter is completely enclosed and 

 correspondingly degenerates to a membranous rudiment. 



In the closely allied Aplysioidea, Dolabclla (fig. 10) still has 

 a calcareous shell with a curved apex, whilst in the large, active, 

 carnivorous sea-hare (Aplysia, fig. u), the nearly enclosed shell 

 is flattened and membranous ; furthermore, in Notarchns the 

 shell is microscopic and the animal is ornamented with dendritic 

 filaments, foreshadowing the cerata or dorsal papillae of the 

 Nudibranchs. According to Bergh, 1 the closely related Asco- 

 glossa (Lobiger, fig. 12 ; Alderia, fig. 13 ; Elysia), in which the 

 shell is usually altogether absent and cerata are more fully 

 developed, form a transitional group between the Tectibranch- 

 iata (as exemplified by the Aplysioidea) and the Cladohepatic 

 Nudibranchiata (/Eolis, Dcndronotus, fig. 14 ; Phyllirrhoe, Tritonia, 

 etc.), whilst he considers the Pleurobranchoidea {Umbrella, 

 Pleurdbranchus), which have developed on parallel lines to the 

 Aplysioidea, to lead on to the Holohepatic Nudibranchiata 

 (Doris, Polycera, Idalia, Phyllidia, etc.). 2 Although both shell 

 and mantle have completely disappeared in the adult Nudi- 

 branchs — the larval stage still possesses a nautiloid shell, as in 

 Aplysia — yet the group is by no means degenerate in character, 

 for it is so universally widespread and shows such a rich 

 variety of adaptive forms, that it must, on the contrary, be 

 regarded as a highly successful branch of molluscs. The dis- 

 appearance of the mantle has enabled the animal to develop 

 cerata to a wonderful extent, with the concomitant advantage, 

 according to Prof. Herdman, 3 either of protective colouring 

 (Tritonia, Doto, Archidoris, Calma) or, when brightly coloured, 

 of acting as warning signals (sEolis, with stinging-cells) ; in 

 his opinion the cerata are only of secondary importance so 

 far as respiration is concerned. 



Whilst the sea slugs or Nudibranchs mark the limit in one 

 direction of the evolution of completely naked forms from 

 primitive Tectibranchiata with massive shells, thereby attaining 

 a greater activity in crawling (only a few, e.g. Phyllirrhoe, are 



1 Semper's Reiscn im Archipelago der PJulippinen, 1870-92. 



8 The Cladohepatic sea-slugs are distinguished by the possession of jaws and 

 a branched liver, the branches often extending into the cerata. In the simpler 

 Holohepatic division the liver is never branched and jaws are usually absent. 



3 Quart. Joum. Microsc. Soc, N.S., xxxi. (1890) p. 41 ; and also Garstang, 

 Conchologist, ii. p. 49, and Hecht, Comptes Rendus, cxv. p. 746. 



