196 THE MICROSCOPE AND ITS REVELATIONS. 



more parts may be practised with a like result. In fact, the power of 

 the Planariae to reproduce portions which have been removed, seems but 

 little inferior to that of the Hydra ( 515); a circumstance which is pecu- 

 liarly remarkable, when the much higher character of their organization 

 is borne in mind. They possess a distinct pair of nervous ganglia (f, /), 

 from w'hich branches proceed to various parts of the body; and in the 

 neighborhood of these are usually to be observed a number (varying from 

 2 to 40) of ocelli or rudimentary eyes, each having its refracting body or 

 crystalline lens, its pigment-layer, its nerve bulb, and its cornea-like bulg- 

 ing of the skin. The integument of many of these animals is furnished 

 with ' thread-cells ' or ' filiferous capsules,' very much resembling those 

 of Zoophytes ( 528). 



594. ANNELIDS. This Class includes all the higher kinds of Worm- 

 like animals, the greater part of which are marine, though there are 

 several species which inhabit fresh water, and some which live on land. 

 The body in this class is usually very long, and nearly always presents a 

 well-marked segrneiital division, the segments being for the most part 

 similar and equal to each other, except at the two extremities; but in 

 the lower forms, such as the Leech and its allies, the segmental division 

 is very indistinctly seen, on account of the general softness of the integu- 

 ment. A large proportion of the marine Annelids have special respiratory 

 appendages, into which the fluids of the body are sent for aeration; and 

 these are situated upon the head (Fig, 407), in those species which (like 

 the Serpula, Terebella, Sabellaria, etc.) have their bodies inclosed by tubes, 

 either formed of a shelly substance produced from their own surface, or 

 built up by the agglutination of grains of sand, fragments of shell, etc.; 

 whilst they are distributed along the two sides of the body in such as 

 swim freely through the water, or crawl over the surfaces of rocks, as is 

 the case with the Nereidce, or simply bury themselves in the sand, as the 

 Arenicola or ' lob-worm.' In these respiratory appendages the circula- 

 tion of the fluids may be distinctly seen by Microscopic examination; 

 and these fluids are of two kinds, first, a colorless fluid, containing 

 numerous cell-like corpuscles, which can be seen in the smaller and more 

 transparent species to occupy the space that intervenes between the outer 

 surface of the alimentary canal and the inner wall of the body, and to 

 pass from this into canals which often ramify extensively in the respira-- 

 tory organs, but are never furnished with a returning series of passages, 

 and second, a fluid which is usually red, contains few floating parti- 

 cles, and is inclosed in a system of proper vessels that communicates 

 with a central propelling organ, and not only carries away the fluid away 

 from this, but also brings it back again. In Terebella we find a distinct 

 provision for the aeration of both fluids; for the first is transmitted to 

 the tendril-like tentacles which surround the mouth (Fig. 407, b, b), 

 whilst the second circulates through the beautiful aborescent gill-tufts 

 (k, k), situated just behind the head. The former are covered with 

 cilia, the action of which continually renews the stratum of water in 

 contact with them, whilst the latter are destitute of these organs; and 

 this seems to be the general fact as to the several appendages to which 

 these two fluids are respectively sent for aeration, the nature of their 

 distribution varying .greatly in the different members of the class. The 

 red fluid is commonly considered as blood, and the tubes through which 

 it circulates as blood-vessels; but the Author has elsewhere given his 

 reasons 1 for coinciding in the opinion of Prof. Huxley, that the colorless 



1 See his " Principles of Comparative Physiology," 4th Edit., 218, 219, 292. 



