106 THE INVERTEBRATE PHYLA 



the major groups there is some disagreement. Thus Raven in his 

 account of morphogenesis in the Mollusca states that it is incorrect 

 that the group as a whole shows determinate cleavage. This would 

 indicate that the Mollusca-Annelida link is not necessarily as 

 close as some authors imagine. On the other hand it is necessary 

 to keep some sense of balance and not lose sight of the wood 

 because of the trees. 



Grobben's classification is shown in Fig. 36. In some ways it 

 resembles the next classification to be discussed, that of Marcus, 

 but there are certain differences. Thus the Coelomata arise from 

 the line that led to the Ctenophora. The Enteropneusta are more 

 allied to the Echinodermata than they are to the Tunicata or 

 Vertebrata. 



Marcus's Classification 



This view of the phylogeny of the invertebrates has been 

 described by Marcus (1958) and it agrees in many ways with that 

 described by Grobben and also with that described by Ulrich 

 (1950) and Remane (1954). Marcus considers that the Anthozoa 

 are the most primitive of the Coelenterata. All the forms above 

 the Coelenterata are called " Bilateria " since they are almost all 

 bilaterally symmetrical. They are also called " Coelomata " and 

 Marcus considers that all these forms are derived from an 

 ancestor that had the " fundamental features of the Archicoelomata, 

 viz. three coeloms, mouth, anus, vessels and perhaps tentacles." 

 The coelom was developed as a series of pouches from the gut, as 

 suggested by Sedgwick (1884), and the Bilateria could have arisen 

 from either the Anthozoa or the Ctenophora. 



Since the Bilateria are all coelomate this means that the 

 Platyhelminthia, Rotifera, Nematoda and Endoprocta all are 

 derived from a form that once had a coelom. During the course 

 of evolution the coelom became reduced in these forms till some- 

 times all that is left is the cavity of the flame cells. The resemblance 

 that has been reported between the planula larva and the Acoela, 

 Marcus suggests, is entirely due to the small size of the animals. 



The Bilateria are divided by Marcus into the Protostomia and 

 Deuterostomia, as we have already seen in Grobben's classification, 

 the only addition being the newly described Pogonophora, which 

 are placed in the Deuterostomia. Marcus points out that there 



