VIII 



PHYLUM MOLLUSCOIDA 



343 



from the mesozoic, and twenty-one in the cainozoic and recent 

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

 extinction. 



Recent researches on fossil and recent forms have shown the 

 Brachiopoda to illustrate, in a remarkable manner, the Recapitu- 

 lation theory already referred to : the theory, that is, that 

 ontogeny or individual development 

 is a more or less modified recapitula- 

 tion of phylogeny or ancestral develop- 

 ment. 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 sim- 

 plest to those with the most complex 

 loop. 



Iph 



MUTUAL RELATIONSHIPS OF THE 

 CLASSES OF THE MOLLUSCOIDA. 



mth 



d.gl 



FIG. 274. Lophophore of embryo 

 of Terebratulma. it. </!. di- 

 gestive gland ; int. intestine ; 

 (p. lip ; Iph. lophophore ; mth. 

 month. (From Korschelt and 

 Heider, after Morse.) 



In adult structure Phoronis ex- 

 hibits marked resemblances to the 

 Ectoprocta, more especially to the 

 Phylactolsemata resemblances which 

 will be rendered clear by a compari- 

 son of the diagrams A and B of Fig. 

 275. 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 lopho- 

 phore, the presence of an epistome having similar relationships in 

 the two groups, and the fact that the coslome is similarly developed 

 in the two groups, point in the same direction. Some points 

 which are supposed to indicate relationships with the Annulata 

 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 of the 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. 274) has the horseshoe shape which it preserves in 

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

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



