THE INVERTEBRATE PHYLA 105 



perforates the embryonic rudiment and that in some of them the 

 anus is directly derived from it. (Many Pisces, some Amphibia, 

 e.g. Newt.) Whereas in others not at all remote from these, the 

 blastopore closes entirely and the anus is a new formation (some 

 Pisces and Amphibia, e.g. Frog, Amniota). Here also we think 

 it may be fairly maintained that notwithstanding the diversity in 

 the mode of development of the anus, it is, in all vertebrata at 

 least, a derivative of the blastopore." 



It will be seen from Sedgwick's account that whilst he realised 

 there was considerable diversity in the fate of the blastopore, he 

 thought the generalisation — that the blastopore became the 

 mouth in the Protostomia and the anus in the Deuterostomia — a 

 fair one. On the other hand Manton (1948) thinks that the 

 variability in the mode of development of the mouth and anus is so 

 great in the Annelida and the Arthropoda that it no longer forms a 

 useful link between these two groups. " It is clear that most of the 

 known species of Onychophora fall into line with the Arthropoda 

 in the dissociation of the mouth and anus from the blastoporal 

 area and contrast with the majority of the Polychaeta." 



There are other characters that can be used to separate the 

 Protostomia and the Deuterostomia, or the Annelid and the 

 Echinoderm Superphylum as they are sometimes called. 



Annelid Superphylum Echinoderm Superphylum 



Spiral cleavage Radial cleavage 



Blastopore = mouth Blastopore = anus 



Schizocoelic coelom Enterocoelic coelom 



Determinate cleavage Indeterminate cleavage 

 Nervous system delaminates Nervous system invaginates 



Ectodermal skeleton Mesodermal skeleton 



Trochosphere larva Pluteus type larva 



To each and every one of these characters many exceptions 

 can be found and in particular, certain groups of animals seem to 

 lie between the two superphyla. Thus the Brachiopoda have their 

 blastopore forming the mouth, their coelom is enterocoelic and 

 their cleavage is of the radial type. The situation is most difficult 

 for the Nematoda, Ectoprocta and Phorodinea and these are the 

 groups that one would most like to place accurately. Even within 



