Phylogeny of Animals 5^25 



gland of rotifers is continued in development, and becomes the 

 glandular mass on the front region of the foot in many genera 

 of molluscs, alike fresh-water and marine. This exudes a 

 viscous mucilage, which has been shown by Cunningham to 

 be used in formation of the tough egg-cases. The foot in 

 most rotifers ends in a bilobed process, and it is at least of 

 interest to observe that in fresh-water molluscs like Limncea 

 and Succinea, as well as in the marine limpet, it arises as a 

 paired structure. 



The shell-secreting dorsal gland of gasteropods is a structure 

 that early appears and becomes very conspicuous in the larva. 

 Below (p. S'iQ) we refer to its possible homologue in rotifers, 

 when discussing lamellibranch structure. 



The arrangement and finer histology of the excretory tubes 

 in both classes may be said to be identical, while the similarity 

 of the sexual organs and their accessory parts need merely 

 be stated. 



If now inquiry be made as to whether any moUuscan group 

 'permanently exhibits only slight advance on rotiferan structure, 

 such is unquestionably furnished by the Scaphopoda or tusk- 

 shells. These, while exhibiting both molluscan and rotiferan 

 characters, agree with the latter in having no heart or distinct 

 blood vessels, as well as no gills. The corpusculated blood- 

 fluid is retained in haemal sacs at different regions of the body. 

 There is no indication that such negative characters, in what 

 is ordinarily classed as a molluscan animal, are connected with 

 degradation processes, for progressive modification through- 

 out is observed in it from the trochophore stage onward. 



The trochophore and early veliger stages of marine lamelli- 

 branchs or Pelecypoda agree exactly with rotiferan conditions 

 above noted. But all distributional, developmental, and 

 palseontological evidence would tend to show that while still 

 in the early veliger stage of evolution the bivalved molluscs 

 branched off from a fresh-water-derived but marine rotifer- 

 protomolluscan ancestry, and long evolved in the sea, only 

 later, and probably during the devonian epoch remigrating 

 into fresh waters to produce there the "fresh- water bivalves." 



Perhaps the first feature here deserving of comparison is 

 the origin and relation of the lateral shells. The lorica of 

 rotifers, as already indicated, seems most commonly to con- 

 sist of a larger arcuate dorsal and of a smaller — often flat — 

 ventral chitinous plate. But such genera as ColuruSy Monura, 

 and Mytilia show instead two lateral plates often hinged to- 

 gether along the dorsal edge. Again in MetopicUa a dorsal 

 and two lateral elements seem to fuse to form the lorica, while 



