Animals and the Elements in which they Live. 199 



tigations upon Ascidia have satisfied me that Bryozoa, com- 

 pound, and simple Ascidia, form a natural series of well-con- 

 nected types leading to the true Acephala among ordinary 

 Mollusca, among which Bryozoa will form a natural group 

 of compound animals, bearing the same relation to the ordi- 

 nary bivalve shells that common corals bear to the simple 

 Actinia) and Fungioe. Though the doubts entertained about 

 the Foraminifera among Bryozoa, would not affect at all the 

 points under discussion, I may as well state at once, that I 

 have arrived at the conclusion that Foraminifera constitute 

 the lowest type of Gasteropoda, and exemplify, under per- 

 manent forms, the state of division of their germs in their 

 embryonic development. Thus circumscribed, the class of 

 Polypi constitutes a very natural group, containing only 

 animals of an identical radiated structure, the organization 

 of which is at present very satisfactorily known. 



The class of Medusae has been from the beginning so well 

 characterised, and circumscribed within so natural limits, 

 that it has undergone since its establishment only slight mo- 

 difications by the removal of some few genera ; and after the 

 position of the so-called Hydroid Polypi among them shall 

 have been generally acknowledged, I believe it will undergo 

 scarcely any new changes in its extension, though we may 

 still expect extensive improvements, which are indeed very 

 much needed, in the characteristics and internal arrange- 

 ment of their natural families. Considering their structure, 

 the Medusae rank immediately above Polypi. 



The intestinal worms have long been placed among Ra- 

 diata, and considered as a natural class in this gi»eat type of 

 the animal kingdom, notwithstanding so many striking dif- 

 ferences in the plan of their structure. This position was 

 assigned to them upon the ground of the radiated arrange- 

 ment of parts around the head, and the vascular form of some 

 of their genera, and also upon the supposed want of a ner- 

 vous system in all of them. But since the discovery of nerves 

 in all of their types, and since the most intimate relations 

 have been discovered between them and so many other ex- 

 ternal worms, their complete separation from Annelides as a 

 distinct class is hardly recognised now by any modern inves- 



