192 MANUAL OF ZOOLOGY. 



veloped ; and there exists, in even greater numbers, the same 

 series of lateral sacculi or " segmental organs " which we have 

 seen in the Leeches, and which have either a respiratory or a 

 renal function. In all the Oligoctmta the segmental organs 

 communicate internally with the perivisceral cavity as well as 

 externally with the outer medium. A portion of the segmental 

 organs is ciliated, and in all cases the segmental organs of 

 certain of the segments have the special function of acting as 

 efferent ducts for the generative organs. 



Of the little Ndididcz, the. most familiar is the Tubifex rivu- 

 lorum, which is of common occurrence in the mud of ponds 

 and streams. It is from half an inch to one inch and a half in 

 length, and of a bright-red colour. The pseudo-haemal system 

 is provided with two contractile cavities or hearts ; and there 

 is present the same system of lateral tubes, opening externally 

 by pores, as occurs in the Earth-worms. 



The NdididcB are chiefly noticeable on account of the sin- 

 gular process of non-sexual reproduction which they present 

 before they attain sexual maturity. In this process the Ndis 

 throws out a bud between two rings, at a point generally near 

 the middle of the body. Not only is this bud developed into 

 a fresh individual, but the two portions of the parent marked 

 out by the budding point likewise became developed into 

 separate individuals. The portion of the parent in front of the 

 bud develops a tail, whilst the portion behind the bud develops 

 a head. Prior to the detachment of the bud, other secondary 

 buds are formed from the same segment, each in front of the 

 one already produced ; and in this way, before separation takes 

 place, a chain of organically connected individuals is produced, 

 all of which are nourished by the anterior portion of the primi- 

 tive worm. Besides their non-sexual reproduction, the Ndididce 

 possess generative organs when adult, and exhibit true sexual 

 reproduction. With the development of the generative organs, 

 a new segment is added to the body, and certain other modifi- 

 cations take place ; so that the process of attaining sexual ma- 

 turity is actually attended with a species of ntetamorphosis. 



Order III. Tubicola {Cephalobranchiatd). — The Annelides 

 which are included in this order inhabit tubes, which may be 

 calcareous, and secreted by the animal itself, or may be com- 

 posed of grains of sand or pieces of broken shell, cemented 

 together by a glutinous secretion from the body. The body- 

 rings are mostly provided with fasciculi of bristles set upon 

 lateral foot-tubercles or parapodia, by means of which the 

 animal is enabled to draw itself in and out of its tube. The 

 alimentary canal is loosely attached to the integument. The 



