Phylogeny of x'Vnimals 523 



series, in which twisting of the body and ahmentary canal 

 became most pronounced. With this, condensation into one 

 unilateral excretory tube occurred, and into one branchia 

 or gill that fused with the dorsal mantle expanse. The fresh- 

 water representatives became, we would consider, the pecti- 

 nibranchiate Paludinida and Melaniida, the marine forms gave 

 rise to an abundance of genera often with hea\'y shells. 



The Pteropoda and Amphineura will be dealt with sub- 

 sequently. 



A more detailed examination of some developmental points 

 in the above can now be made. The preoral area in most 

 gasteropods forms two eyes and four to two antennae or ten- 

 tacles as in many rotifers. The eye in the latter class may be 

 a simple pigmented depressed cell, or in front of the depression 

 a clear lens may be formed (169, 1: 91; //; 133). In gastero- 

 pods both of these primitive conditions may occur, but alike 

 the pigmented or retinal cell and the lens cell undergo numerous 

 divisions. iVdvance to greatly more specialized eyes is also 

 noted. Four, or three by fusion of two, or two antennae only 

 may be met with in Rotifera, each also provided with special 

 nerve, and motile as well as retractile, according to the graphic 

 account of Hudson-Gosse. Further while usually provided 

 with terminal cirrhi or chaetae, some genera have soft rounded ten- 

 tacular antennae. Like conditions exist in various Gasteropoda. 



The very characteristic mastax or pharyngeal sac of rotifers 

 with its associated teeth has already been referred to in corre- 

 lation with the jaws and paragnaths of the Annelida. But 

 a most interesting correlation and evolution can be studied 

 in passage to the gasteropods. The masticatory system in 

 Rotifera consists typically of a median often bilobed plate, 

 the incus, each half of which is a ramus. This, by slipping 

 forward, may be the homologue amongst pulmonale molluscs 

 of the single scraping jaw seen in Helix, Limax, etc., which, 

 as Wiegmann has shown, is composed of two pieces in the 

 young that fuse in the adult. On either side of the incus 

 are paired plates, at times with serrated edges, in the Rotifera, 

 each of which is known as an uncus. These are found in all 

 ])ulmonates, and at times equal in size to each ramal half of 

 the incus as in Limncea. External to and attached to the 

 margins of the two unci are oblic^ue or longitudinal })ieces, 

 the manubria. These seem to be represented in Limncea 

 by the oblique outer pieces of the unci with wliich they may 

 have become fused. In prosobranch molluscs there are paired 



