264 MANUAL OF ZOOLOGY. 



Of the little Nctidida, the most familiar is the Tubifex riva- 

 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 pseudohaemal 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 Na'ididce are chiefly noticeable on account of the singular process of 

 non-sexual reproduction which they present before they attain sexual ma- 

 turity. 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 devel- 

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

 out by the budding point likewise become developed into separate indivi- 

 duals. 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 primitive worm. Besides their 

 non-sexual reproduction, the Na'ididtz possess generative organs when adult, 

 and exhibit true sexual reproduction. With the development of the gen- 

 erative organs, a new segment is added to the body, and certain other modi- 

 fications take place ; so that the process of attaining sexual maturity is 

 actually attended with a species of metamorphosis. 



As regards their distribution in space, the Oligochata have 

 a cosmopolitan range. The most are either terrestrial, or in- 

 habit fresh waters, burrowing in miTd or sand; but a few are 

 marine. 



ORDER III. TUBICOLA (Cephaicbranckiata}. Animal pro- 

 tected by a tube; locomotive organs in the form of foot-tubercles, 

 carrying set CK ; breathing-organs in the form of branchicz carried 

 on or near the head. Sexes almost always distinct. A metamor- 

 phosis in development. The Annelides which are included in 

 this order inhabit tubes, which may be calcareous, and secreted 

 by the animal itself, or may be composed 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 para- 

 podia, by means of which the animal is enabled to draw itself 

 in and out of its tube. 



The Tubicola are often united with the order of the Errantia 

 under the name of Polycfmta ; and though collectively spoken 

 of as " Branchiate Annelides," they do not always possess 

 specialised respiratory organs. 



The protecting tube of the Tubicolous Annelides may be 

 composed of carbonate of lime (Serpula), of grains of sand 

 (Sabellarid), or of sand, pieces of shell, and other adventitious 



