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PHYLUM MOLLUSCOIDA 



365 



mtli 



mation. Altogether 106 genera are known from the Palaeozoic rocks, 

 34 from the Mesozoic, and 21 in the Cainozoic and Recent periods. 

 Obviously the group is tending, though slowly, towards extinction. 



Researches on fossil and recent forms 

 have shown the Brachiopoda to illus- 

 trate, in a remarkable manner, the 

 recapitulation theory already referred 

 to : the theory, that is, that ontogeny or 

 individual development is a more or less 

 modified recapitulation of phylogeny or 

 ancestral development. It has been 

 shown that there is a striking and 

 almost complete parallelism between the 

 stages in the development of the shelly 

 loop in such highly organised forms as 

 Magellania, and the entire series of 

 articulated Brachiopods from those with 

 the simplest to those with the most 

 complex loop. 



MUTUAL RELATIONSHIPS OF THE 

 CLASSES OF THE MOLLUSCOIDA. 



In adult structure Phoronis exhibits 

 marked resemblances to the Ectoprocta, 

 more especially to the Phylactola3mata 

 resemblances which will be rendered clear by a comparison of 

 the diagrams A and B in Fig. 306. In both, the ventral side of the 

 body is greatly produced and elongated, and, by the approximation 

 of the mouth and anus, the dorsal surface is reduced to a very short 

 space between those two apertures. The form of the lophophore, 

 the presence of an epistome having similar relationships in the two 

 groups, and the fact that the coelome is similarly developed in 

 both, point in the same direction. Some points which are supposed 

 to indicate relationships with the Annulata (Sipunculoidea) and 

 with the Chordata are referred to at a later stage. 



The resemblances between the Brachiopoda and the other two 

 classes of the phylum are somewhat disguised by the development 

 of the shell, but are very obvious more particularly when we 

 take into account certain features of the development. One of the 

 most striking points of resemblance between the three classes 

 is the presence of the lophophore with its tentacles ; in the earlier 

 stages of its development in the Brachiopod, as we have seen, this 

 structure (Fig. 305) has the horse-shoe shape which it retains in 

 the adult Phoronida and Phylactolsemata, and* a lobe the arm- 

 fold or lip (Ip) comparable to the epistome, is present overhanging 

 the mouth. The end of the body of the Brachiopod with which 

 the peduncle is connected must correspond to the aboral extremity 



FIG. 305. Lophophore of embryo 

 of Terebratulina. d. gl. di- 

 gestive gland ; int. intestine ; 

 Ip. lip ; Iph. lophophore mth. 

 mouth. (From Korsehelt and 

 Heider, after Morse.) 



