29 



turn out to be the case, the common origin of the Chordata, 

 the Enteropneusta, and the Echinodermata (for these three 

 groups are undoubtedly related) must be moved back on the 

 main axis so as to be at a different point from the origin of 

 the Mollusca, and nearer to the Gastrea.* 



Turning now to the branch representing the phylum 

 Molluscaf we find that no existing forms are very primitive. 

 Probably the most nearly related to *he ancestral Molluscs 

 are those composing the group Isopleura, but even they are 

 very considerably differentiated. To express this a long line 

 has been left between the origin of the Molluscan branch and 

 the first point of division into the ancestral forms of the 

 existing groups of the Mollusca. 



Professor Kay Lankester has reconstructed the probable 

 common ancestor of the Mollusca, the form which occupied 

 the point of division of the branch into primitive Isopleura, 

 Lamellibranchiata, and Cephalophora. This Archi-Mollusc 

 (fig. 15) is bilaterally symmetrical and elongated antero- 

 posteriorly. It has a flat ventral and a rounded dorsal 

 surface. The head is well marked, and has a region in 

 front of the mouth (prostomium) on the upper surface of 

 which are placed a pair of cephalic tentacles. The mouth 

 is placed in the middle line anteriorly, but behind the 

 prostomium. The anus is placed in the middle line pos- 

 teriorly, and rather on the dorsal surface. A pair of renal 

 excretory organs (nephridia) open one at each side of the 

 anus, while close to the nephridial apertures, but further 

 forward, are the openings of the paired reproductive organs. 

 The ventral surface of the body is formed of a thick muscular 



* Balfour advocates the view that the Echinodermata at least have 

 sprung from a radially symmetrical ancestor, and that the bilateral, 

 symmetry of the larval forms is secondary {Comp. Emb., vol. ii, p. 318.) 



f The student is referred to Lankester's article ** Mollusca," Ency. Brit. 

 9th ed., p. 632. 



