64 



MOLLUSC A. 



Fig. 47, the result is a fair presentation of the Lamelli- 

 branch. Imagine this bag or mantle in one piece, 

 slightly conical in form, the foot changed to a broad, 

 flat, crawling disc, and the gills arranged in rows of 

 plates on the side, as in Fig. 50, and the result is Patella, 

 a Gasteropod, of which a section is shown in Fig. 51. 

 Elongate the cone and shell, twisting it into a spiral, 

 and carry up the fold at the mantle border into a 

 deeper pouch, protecting the breathing organs, and we 

 have a coarse model of the Marine Snail (Fig. 53). 

 Reduce the cone in size to a mere patch on the back 

 of the foot, or finally abolish it altogether, and we 

 have first Limax (Fig. 54), and then the Nudibranch* 

 (Fig. 28). 



* This is, therefore, a form which has retrograded or lost 

 the normal characteristics of its type more even than the Limax, 

 since one region of the body has wholly disappeared. To this 

 may be added another curious but instructive instance. There 

 are but few parasitic Mollusca, but among them is one originally 

 supposed to be a worm, and found in the interior of Holothuri- 

 ans (Huxley's Anatomy of Invert., p. 440.). This can be truth- 

 fully said to present none of the characteristics of the type in the 

 adult, but the embryo is a true Mollusc, having even the shell gland 

 and shell. No other inference is here possible, except that the 

 mode of Hfe, as a parasite, has effected this remarkable retro- 

 gression by which the animal in course of growth has lost that 

 which fitted it to live in the external world, and changed so com- 

 pletely as to present a singular resemblance to a worm. I say a 

 resemblance, because it is not changed into a worm, but into the 

 close semblance of one; for the worms themselves have a plan 

 of structure of their own, to which this cannot be referred. The 

 illustration is brought forward merely to show the futility of 

 using word-bound definitions, since all structures are liable to the 

 most fundamental changes whenever the conditions of the sur- 

 roundings of existence are fundamentally changed. 



