234 SYNOPTIC COLLECTION. 



condition. But inasmuch as this torsion is greater in the 

 embryo than in the adult, the less degree of twisting is a 

 reduced rather than a primitive characteristic. The asym- 

 metrical condition of the heart and the nephridium prove 

 that the bilateral symmetry of the adult is also a second- 

 arily acquired and not a primitive condition. Gilchrist 1 

 has shown that, while the original bilateral symmetry is 

 apparently assumed, the organs which were lost on one 

 side in the twisting of the straight body into a spiral are 

 never again developed. 



Not only has the Nudibranch lost its shell, but in some 

 species the mantle has disappeared. 



In other ways this group has become peculiarly special- 

 ized. Many Nudibranchs are without true tentacles, while 

 most have organs called dorsal tentacles or rhinophores 

 which are supposed to be organs of smell. The branchiae 

 are not like the gills of most molluscs, but are secondarily 

 acquired, respiratory organs, and are often called cerata 

 to distinguish them from the primitive gill. These bran- 

 chiae vary greatly both in shape and position and are 

 often extremely beautiful organs. 



Eolis ( = Aeolis) coronata Forbes lays its eggs in a 

 close-set spiral coil of four volutions (PI. 518, fig. i ; fig. 

 2, a portion of the same magnified, showing the eggs 

 imbedded in a gelatinous thread). The larva (fig. 3) is 

 provided with a spiral shell (fig. 4) , which is closed by 

 an operculum. At this stage the larva has the two ciliated 

 oral lobes which aid in locomotion. With the growth 

 of the animal the shell and operculum disappear and the 

 oral lobes are either modified or wholly absorbed. The 

 mature Eolis (fig. 5, upper side; fig. 6, lower side; No. 

 519, JS. papillosa Loven), has a long slug-like body wholly 

 unprotected by a calcareous or horny shell. The forward 

 part bears a pair of tentacles, and also a pair of rhinophores 

 or dorsal tentacles. Along the back the brilliantly col- 



1 Proc. Roy. Soc. Edinburgh, XX, 1895, p. 368. 



