APPENDIX. 377 



is something fundamental to all these conditions of its exhibition. The 

 fact of the radiate structure, and of the general homology in the several 

 parts between the Eohinoderms and other Radiates, is not affected by the 

 fact of the nutritive system having one or fr\vo open extremities, or by 

 the perfection of the nervous or branchial systems, or by the condition 

 of the general visceral cavity. Moreover some Echinoderms have only 

 one opening to the alimentary cavity, while some Acalephs have two, like 

 the highest Echinoderms, thus proving that such distinctions are of small 

 importance alongside of system of structure. Again, the nervous system 

 of Echinoderms, as already stated, is only the perfected state of the ner- 

 vous system of some Polyps and Acalephs. 



Echinoderms appear to differ strikingly from Polyps in having 

 many tentacles from one tentacular compartment. But in Polyps, one 

 compartment has occasionally, besides its one tentacle, a series of them ; 

 thus evincing the same fundamental idea in the structure of the two, and 

 affording proof of their close relationship. The branchial rosette in a 

 Holothurian looks quite peculiar ; but Actinia? that live, like most Holo- 

 thurians, in the sand, have sometimes a similar branchial rosette, the 

 crimped or finely divided appendages among the tentacles of such Actiniae 

 being true branchire, as Verrill has observed; and, further, such appen- 

 dages have no compartment of their own, but grow out from one that 

 bears its normal tentacle (page 39). The group of tentacles and branchial 

 appendages in the Actinia constitute a rosette around the mouth wholly 

 analogous to that of a Holothurian. This peculiarity is therefore con- 

 firmatory evidence that Polyps and Echinoderms are one in system of 

 structure and alike Radiates. 



III. PROTOZOANS. 



Foraminifers, which include the Orbitolites (mentioned on page 152), 

 the Globigerinse (page 212), and also Sponges, are the secretions of 

 Protozoans, just as ordinary corals are the secretions of Polyps. 



Protozoans, the lowest and simplest of animals, show their simplicity in 

 first, their minuteness, the animals being mostly between a 100th and a 

 10,000th of an inch in length ; secondly, in having no external organs or 

 parts, excepting (1) a mouth, and (2) minute cilia or thread-like processes ; 

 thirdly, in having no distinguishable digestive apparatus excepting a 

 stomach ; fourthly, in the fact that the stomach and mouth are some- 

 times wanting, or exist only when extemporized for the occasion. The 

 species have, besides, a palpitating vesicle or vacuole within the body 

 which appears to serve the purpose of a heart. Part of the so-called 

 Infusoria are Protozoans. 



In the lowest section of Protozoans — that of the Hhizopods — the 

 animal has a spheroidal body, if of any particular shape, but is generally 



