METAZOA MOLLUSCA. 233 



spiral is seen within. Every whorl entirely conceals the 

 preceding, as we have seen it in Ultimus (No. 492). 



The "sea-hare," Aplysia (No. 514, A. limacina; No. 

 515, A. inca d'Orb.) has only the remnant of a shell on its 

 Sack in the form of a concave plate, and this is concealed 

 by a fold of the mantle. On the dorsal side the mantle 

 grows out into two large folds, called epipodia (see Nos. 

 514, 515) which are used in swimming. The single 

 branchia is on the back protected by the mantle. The 

 model (No. 515) represents the animal as crawling on its 

 long ventral foot. Garstang 1 has pointed out the interest- 

 ing fact that young Aplysiae in migrating during growth 

 from deep water to the shore pass through algae colored 

 first red, then brown, and finally olive green, and that 

 these animals change their colors from red to green in 

 accordance with their surroundings. 



The shell is reduced and concealed by the mantle in 

 Pleurobranchus (No. 516, -P. grandis Pease). Lobiger 

 (No. 517, L. picta Pease), has a small shell on its back and 

 two pairs of wing-like swimming organs extending from 

 the sides of the body. 



NUDIBRANCHS. 



The Nudibranchs pass through a trochophore and veli- 

 ger stage in their development. They possess a spiral or 

 nautiloid shell before they leave the egg, and the body at 

 this time is more or less spiral in form. During the growth 

 of the larva the shell disappears and the animal extends 

 lengthwise. The possession of a shell in the embryo is 

 evidence that the Nudibranchs have descended from 

 Gastropods with spirally coiled shells. According to 

 Kingsley 2 the twisting or torsion of the body has not been 

 carried to its full extent, which would indicate a primitive 



1 Journ. Mar. Biol. Assoc., n. s., I, 1889-90, p. 411. 



2 Stand. Nat. Hist., I, 1885, p. 295. 



