90 THE MOST PRIMITIVE METAZOA 



Resemblances between Coeloplana and the Polycladida 



(1) Both have a flat, compressed body. 



(2) They move by creeping on the sole of the " foot." 



(3) The body surface is ciliated. 



(4) There is a well- developed basement membrane. 



(5) The dermal musculature is well developed; the dorso- 

 ventral muscles may be branched. 



(6) The gastric canals have many branches; there is no anus 

 but some pores end externally. 



(7) There is a stomodeal invagination on the ventral surface. 



(8) Both show determinate cleavage. 



(9) The large micromeres give rise to small micromeres. 



(10) The micromeres form the ectoderm. 



(11) There is no hollow blastula stage. 



(12) The development is mosaic. 



(13) The mesoderm arises from the macromeres. 



(14) The embryo gastrulates by epiboly. 



(15) The Muller larva present in some polyclads has eight 

 ciliated lappets. 



(16) There is a small apical nervous tuft. 



(17) There is a statolith. 



(18) Both groups have paired tentacles. 



(19) There are gonadial canals. 



The above list is impressive in length and indicates a consider- 

 able similarity between the two groups. Lang suggested that the 

 centrally positioned nerve centre in the ctenophores moved 

 anteriorly to take up the typical polyclad position, i.e. a change 

 from biradial symmetry to bilateral symmetry (Fig. 32). The list 

 may also demonstrate another point. Often in discussing such a 

 problem, a list of characters is drawn up of the points for and the 

 points against a given viewpoint. The difficulty comes when one 

 has to decide the relative importance of each character. It is 

 impossible to come to a decision just by seeing whether there are, 

 say, more similarities than differences. Instead each point must be 

 weighted according to its importance and this is difficult since the 

 importance often reflects the opinion of the observer. 



In spite of the similarities between Coeloplana and the Poly- 

 cladida, the general opinion these days is that the similarities are 



