SECT. I OBJECT AND LINE OF ARGUMENT 3 



Between the Coelenterata and the Platodes we have 

 rival links. When reading the arguments in favour 

 of the claims of those specialised Ctenophora, the 

 Cceloplana and Ctenoplana, we feel convinced ; but, 

 on the other hand, when we study for ourselves 

 under the microscope such a simple Rhabdoccele as 

 Microstomum lineare, especially during its changes 

 of shape when moving about under a cover glass, 

 our former conviction fades away, and we see in it 

 a specialised larval form of a Ccelenterate. Between 

 the Platodes and the Annelids the gap seems small, 

 but we cannot bridge it over until we decide whether 

 the segmentation of the Annelids is a kind of axial 

 strobilation, or the natural mechanical selection of 

 internal symmetry. Between the Annelids and the 

 Molluscs we have the claims of Solenogaster to attend 

 to ; but this animal is unfortunately so rare, that 

 it will be long before we can hope to have any very 

 thorough knowledge of its morphology. Between the 

 Annelids and the Tracheata we have Peripatus ; this 

 highly interesting animal, has a special claim on our 

 attention, as the Tracheata form with the Crustacea the 

 great class known as the Arthropoda. We shall find 

 that our explanation of the rise of the Crustacea 

 supplies us also with a very probable clue as to the 

 origin of the Tracheata. The Echinodermata and 

 the Tunicata hover almost entirely in the air. And, 

 lastly, we have the giant trunk of the Vertebrata, the 

 roots of which are being eagerly sought in different 

 directions. The claims of Amphioxus and of^the 

 Ascidian larva are confidently put forward by the 



B 2 



