THE MAIN LINES OF ANIMAL EVOLUTION 



a matter of conjecture. Origin from some primitive, acoelous flatworm is 

 probable, yet there is little basis for a decision as to which of the possible 

 groups of flatworms was the ancestor, beyond the fact that spiral cleavage 

 is also found in the Nemertinea and in the polyclad Turbellaria. Because 

 of the widespread occurrence of the trochophore larva, it is generally 

 supposed that an independent, trochophore-type animal was an ancestor 

 intermediate between the flatworms and the present-day phyla of the 

 annelid-arthropod-molluscan series. The only evidence for this is the in- 

 terpretation of the embryological evidence according to the Biogenetic 

 Law, and it has already been pointed out how insecure this is. Yet, al- 

 though it may be true, direct evidence is not likely to be obtained, for 

 such an organism would be very poor material for fossilization, and it 

 would have to be sought in pre-Cambrian rocks, for the major invertebrate 

 phyla were all present in the Cambrian. 



Phylum Mollusca. Whatever the source, the phylum Mollusca was 

 early established as a group divergent from the others. This is a great 

 phylum, the second largest in the Animal Kingdom, having around 80,000 

 species encompassing a great variety of forms, and inhabiting almost 

 every type of marine, freshwater, and terrestrial habitat. All of the extant 

 classes of Mollusca were present already in the Cambrian, and literally 

 hundreds of species are known. The classes were just as distinct then as 

 now, and so paleontology is of no help in deciding what the relationships 

 within the phylum may be. 



In addition to the general characters of all members of the protostome 

 line, the phylum Mollusca is distinguished by many structural characters. 

 In all, the body is divided into four regions: a muscular foot, a head, a 

 visceral hump, and a mantle which generally secretes a calcareous shell. 

 Basically, they are bilaterally symmetrical, but this is obscured in the 

 adults of some classes. The coelom is very much reduced. Mollusca are 

 traditionally described as unsegmented, yet in 1957 Neopili7ia, a new spe- 

 cies from the deep waters of the Gulf of Mexico, was described. This 

 animal has several segmentally arranged organ systems, and its affinities 

 are with a class, Monoplacophora, which had been thought to be extinct 

 since the Devonian. It indicates that the primitive molluscs may have been 

 segmented. On the basis of anatomical evidence, the class Amphineura is 

 regarded as very primitive, but not necessarily as the progenitor of the 

 more specialized classes. This class comprises the chitons or sea cradles, 

 animals which are familiar parts of the fauna of rocky sea coasts, but there 

 are no freshwater species. The chitons have a broad, flat, elongated foot, 

 over which lies the visceral hump and at the anterior end of which is the 

 head. The mantle covers the visceral hump and typically secretes a series 

 of eight calcareous plates, the valves, which may be beautifully orna- 

 mented. 



The class Gasteropoda, which includes the snails, slugs, limpets, and 

 their less well-known allies, has developed a much enlarged visceral 

 hump. In most gasteropods, this hump has grown asymmetrically, with 

 the familiar coiling as a result. Correlated with this, there has been a tor- 

 sion of the visceral hump through 180°, so that structures which were 



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