INTRODUCTION. XIU 



as the completing of some organ or function, even as zoolo- 

 gically it represents the completing of some important 

 modification of form. In the lowest division of the Radi- 

 ata, that of Zoophytes, the digestive system has passed 

 through all its essential changes, therefore we do not see 

 a true progression of the organs of that system among the 

 Echinodermata. We find its variations depend rather on 

 the circumstances under which the species are destined to 

 live than on any progression of structure. Thus it appears 

 anomalous at first, that the lowest Echinodermata should 

 have digestive organs of more complicated nature than 

 some higher tribes ; that the Feather-star should have an 

 intestinal canal with two orifices, whilst all the Ophiuridpe 

 and many true Starfishes have but one ; or that the 

 Splancno-skeleton (dental apparatus) of the Echinidse 

 should be more highly developed than that of the Holo- 

 thuriadse ; but looking at the system according to the view 

 I have taken of its completion in a lower tribe, this is 

 what we should expect. Not so, however, with all the 

 functional systems. The respiratory goes through a series 

 of modifications complicating as we advance, so also do 

 the circulatory and the nervous. But it is the muscular 

 which is especially presented in all its essential modifica- 

 tions in the class before us, from the first appearance of 

 a contractile tissue, as seen in the granular tissue of 

 the lowest Echinodermata, to its perfect developement in 

 the complicated muscular mechanism presented by many 

 of the Vermigrade species. 



The system most characteristic of the Radiate tj-pe is 

 the Aquiferous, or apparatus for a water circulation ; in- 

 deed, it can scarcely be said to exist in any of the other 

 types. It is chiefly developed in the Arachnodermata and 

 Echinodermata, and in the last is intimately connected 



