78 E. RAY LANKESTER. 



condition, then it ceases to be surprising that, with so much 

 • confidence, lowly developed or degraded groups of Mollusca 

 are detached by systematists from the Molluscan tree, and 

 referred back to the fundamental group of Worms. 



There are three modifications of structure which are dis- 

 tinctive of the Mollusca, and are of significance in the order 

 in which they are mentioned, viz. foot, gill, mantle-shell. 

 They each have their representative, their homogen, in the 

 commonest worms, but unspecialised, not elaborated in the 

 Molluscan style. The foot is a muscular specialisation of 

 the ' neural ' or ' ventral ' post-oral body-wall. It is the 

 most characteristic of all Molluscan features. The gills, 

 again, are lateral and post-oral diverticula of the body-wall, 

 such as are found as respiratory organs in many Vermes. 

 But in some classes of the Mollusca they have taken on 

 special functions with accompanying changes of form and of 

 apparent relations, which strangely obscure their origin. The 

 mantle, with the calcareous matter to which it gives origin, is 

 again fully represented in the tubicolous Annelids, and is, 

 after all, a mere flap of the body-wall, which may be de- 

 veloped to an enormous extent or be absent altogether as well 

 as the shell. At the same time the production of an em- 

 bedded chitinous shell (pen of Cephalopoda, embryonic plug 

 of Gasteropoda, ligament of Lamellibranchs), together with 

 a superficial calcareous valve or valves, must have at a very 

 early period of its branching ofl" have become characteristic of 

 the Molluscan pedigree. 



There is a consideration of a general character, which relates 

 to the probable effect of a long-continued process of evolution 

 of organic forms, and is of the greatest importance in the ex- 

 amination of the Molluscan pedigree, as, indeed, in all such 

 inquiries. That consideration is this, that the lowly organised 

 forms which we at present see are hy no means necessarily , 

 though they are possibly, and often, no doubt, are actually, 

 representatives of the lowly organised ancestry of the higher 

 forms to which they are most nearly allied. They oftep, on 

 the contrary, must be degraded forms, or forms which have 

 progressed in the direction of simplification from a more 

 highly elaborated or more * typical ' ancestry. Thus, very 

 probably, both Amphioxus and the Ascidians do not indicate 

 to us the direct way backwards from the Vertebrata to an 

 invertebrate ancestry. They both very probably have become 

 vastly modified and simplified in their own way, as compared 

 with the ancestor common to them and the Vertebrata. 

 Long ago Goethe perceived the tendency to substitute the 

 order in which things become known to us for the order of 



