APPENDIX. 325 



fact of the nutritive system having one or two open extremities, or 

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

 condition of the general visceral cavity. Moreover some Kchino- 

 derms have only one opening to the alimentary cavity, while so)nc 

 Acali'pJis 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 nervous 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 Actiniae 

 that live, like most Holothurians, in the sand, have sometimes a 

 similar branchial rosette, the crimped or finely divided appendages 

 among the tentacles of such Actiniae being true branchiae, as Verrill 

 has observed ; and, further, such appendages have no compartment 

 of their own, but grow out from one that bears its normal tentacle 

 (page 19J. 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 confirmatory 

 evidence that Polyps and Echinoderms are one in system of 

 structure and alike Radiates. 



III. PROTOZOANS. 



Foraminifers, which include the Orbitolites (mentioned on page 

 121), the Globigerinfe (page 174), 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, fii'st. their minuteness, the animals being mostly 

 between a looth and a 1 0,000th of an inch in length ; secondly, in 

 having no external organs or parts, excepting (i) a mouth, and (2) 

 minute cilia or thread like processes ; thirdly, in having no dis- 

 tinguishable digestive apparatus excepting a stomach ; fourtlily, \\\ 

 the fact that the stomach and movith are sometimes wanting, or 

 exist only when extemporized fo)- 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 Rhizopods — the 

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

 generally without a permanent mouth or stomach. It has the 

 power of extending out portions of its protoplasmic body in the 

 form of thread-like processes, and thence the name Rhizopod, 



