372 



ZOOLOGY 



SECT. 



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 



recapitulation of phylogeny or ancestral 

 development. It has been shown that 

 there is a striking and almost com- 

 plete parallelism between the stages in 

 the development of the shelly loop in 

 such highly organised forms as Magel- 

 lania, and the entire series of articu- 

 lated Brachiopods from those with the 

 simplest to those with the most complex 

 loop. 



Iph 



mth 



MUTUAL RELATIONSHIPS OF THE 

 CLASSES OF THE MOLLUSCOIDA. 



FIG. 299. Lophophore of embryo 

 of Terebratulina. <l. gl. di- 

 gestive gland ; int. intestine ; 

 ip. lip ; Ip/i. lophophore ; mth. 

 mouth (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 comparison 

 of the diagrams A and B in Fig. 300. 

 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 direc- 

 tion. 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 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. 299) has the horse-shoe shape which it retains in 

 the adult Phoronida and Phylactolasmata, 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 

 in the Polyzoa, since this represents the part by which the larval 

 Polyzoan becomes fixed, the everted " sucker " of the latter being 



