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, 011 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, (fee.) 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, tfcc. ; : 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 faeNereida . 

 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 



iu. t'O. v_;iiuuiii'Uiiii: uj[jijciji-u>iju ui . -. 



Terebella concMlega : a, labial ring; Species to occupy the space that 



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



the trunk ; <7 skin of the back; e f ace of the a li men tary caiial and 



pharynx; f, intestine ; ff. longitudinal ,, n /? ,1 i , 



muscles of the inferior surface of the the "iner wall of the body, and to 



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



of generation ;j, feet; M, branchiae; often ramify extensively ill the 

 /. dorsal vessel acting as a respiratory . J 



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

 , 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 



FIG. 715. Circulating apparatus of 



gus ; ;;', 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 Jaitrn. Iloij. Micr. Snc. 1890, p. 685. 



