948 



WORMS 



on land. The body in this class is usually elongated and nearly 

 always presents a well-marked segmental division, the segments 

 being for the most part similar and equal to each other, except at 

 the two extremities; though in some, as the leech and its allies, 



the segmental division is very in- 

 distinctly seen, on account of the 

 general softness of the integument. 

 A large portion of the marine An- 

 nelids have special respiratory ap- 

 pendages, into which the fluids of 

 the body are sent for aeration, and 

 these are situated upon the head 

 (fig. 715) in those species which 

 (like the Serpula, Terebella, Sabel- 

 laria, <fcc.) have their bodies inclosed 

 by tubes, either formed of a shelly 

 substance produced from their own 

 surface, or built up by the agglutina- 

 tion of grains of sand, fragments of 

 shell, &c. ; 1 whilst they are distri- 

 buted 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 

 circulation of the fluids may be dis- 

 tinctly seen by microscopic exami- 

 nation ; and these fluids are of two 

 kinds : first, a colourless fluid, con- 

 taining numerous cell-like cor- 

 puscles, which can be seen in the 

 smaller and more transparent 



that 



FIG. 715. Circulating apparatus of 



"Terebellaconchilega: a, labial ring; species to occupy the space 



b, b, tentacles; c, first segment of intervenes between the outer sur- 



the trunk ; d, skin of the back ; e f ace of the alimentary canal and 



pharynx: f, intestine ; q, longitudinal ,-, ,, ,, ., i r , , . 



musclesof the inferior surface of the the inner wall of the body, and to 



body; h, glandular organ; i, organs pass from this into Canals which 

 of generation;./, feet; k, k, branchise ; o f ten ram if y extensively in the 

 L dorsal vessel acting as a respiratory . * J 



heart; m, dorso-intestinal vessel; respiratory organs, but are never 

 n, venous sinus surrounding oesopha- furnished with a returning series 



of passages ; and second, a fluid 

 which is usually red, contains few 

 floating particles, and is inclosed in 

 a system of proper vessels that communicates with a central pro- 

 pelling organ, and not only carries the fluid away from this, but also 

 brings it back again. In Terebella we find a distinct provision for the 



gus ; n', inferior intestinal vessel; 

 o, o, ventral trunk ; p, lateral vascular 

 branches. 



1 For an interesting account of the formation of these tubes see Mr. A. T. Watson's 

 paper in Journ. Boy. Micr. Soc. 1890, p. 685. 



