302 



TtRBELLARIA.- 



-TUEBELLARIA." 



-Teretularia. 



Class— TURBELLARIA (The Planarias, Nemertines, &c.) 



This class of worms was established by Ehrenberg, 

 and has been revised by Oersted. It contains, accord- 

 ing to the arrangement of Dr. Johnston, all these non- 

 jiarasitical worms the body of which is not divided 

 into segments, and which is destitute of bristles on the 

 sides, described in the previous class of Annelides as 

 taking the place of feet. The extremities of the body 

 are simple, and continuous with it, there being no dis- 

 tinct head visible. In form this body is either flat or 

 subcylindrical ; in substance soft, parenchymatous, or 

 cavernous, and in general it is naked and lubricous. 

 Tlie whole body is covered more or less with vibratile 

 cilia or with papilte ; it is often very contractile, 

 assuming a variety of shapes at different times, and 

 tliere are some species which break up voluntarily into 

 pieces. The front portion of the body, the head being 

 imperfectly defined or completely continuous with the 

 body, has in most cases no tenfacula, but in some 

 genera there are two, prolonged from the surface. 

 The mouth in some is terminal, in others ventral ; and 

 in the latter case it is situated in the anterior third, 

 or near the centre, or towards the tail, and is often 

 furnished with a protrusile proboscis. The intestine in 

 one group runs undivided through the length of the 

 body, and in another is dendritically branched ; and in 

 these latter this canal is without a vent. The Turbel- 

 laria are either of separate sexes, or they are herma- 

 phrodite ; most of them propagate by ova, but some 

 iiy transverse fissuration. These worms are animal- 

 feeders for the most part, though there are some which 

 appear to make their diet, in part at least, consist of 

 decaying vegetable matters. In general they are 

 natives of salt or fresh water, though a few are found 

 (n land in moist places. Their mode of progression 

 is by gliding smoothly along the surface of any body 

 upon which they attach themselves, as under stones, on 

 tlie leaves of algae and submerged plants, &c., or by 

 swimming, an act which they execute rather slowly. 

 A few of the species are found taking up their abode 

 in the deserted tubes of other worms, or even, though 

 rarely, forming for themselves a sheath of dried gluten 

 of considerable tenacit}'. 



The class Turhcllaria may be divided into two 

 orders — I. Planaria, containing the true Planarias ; 

 and II. Teretularia, containing those long linear or 

 riband-shaped worms, called Linens, Borlasia, or Ne- 

 mertes, &c. 



Order I.— PLANARIA {The Planarias). 



The body of these worms, as their name would 

 indicate, is ilattened, and usually not much longer than 

 broad. It is covered with a ciliated epithelium, and is 

 of a loose, cellular, parenchymatous structure. A few 

 are destitute of eyes, but the greater number have them 

 pretty well developed, and sometimes even numerous 

 and arranged in clusters. The mouth is a simple pore, 

 and often furnished with a protrusile proboscis. The 



intestine has no vent or posterior opening, and is either 

 in form of an undivided tube, or branched like a 

 shrub. 



Tliis latter character has been taken as a moans of 

 dividing the order into two sub-orders or large groups 

 — R/iabdoccela, and Dcndroccela. 



Sub-order I.— RHABDOCtELA. 



Alimentary canal simple and C3'lindrical ; no pro- 

 trusile proboscis ; locomotion mostly natatory. 



The animals of this group are small, aquatic crea- 

 tures, living both in fresh water and salt. The body 

 is of a parenchymatous consistency, and it is often 

 difBcult to trace any distinctly-defined viscera or their 

 openings. It is contractile to a certain extent, and has 

 an entire margin. There is no distinct head, and in 

 some the eyes are altogether wanting, whilst in others 

 they occur only two in number. Tlie mouth is either 

 terminal or nearly so. The fresh-water species pro- 

 pagate by ova, and these are laid inclosed in a cocoon 

 or capsule. The marine species are less known. A 

 good many species are found in this country ; and one 

 genus has been named DalyelUa, in compliment to 

 Sir John Dalyell, the author of a very interesting account 

 of British Planaria. 



Sub-order II.— DENDROC^LA. 



Intestinal canal dendritically ramified; a protractile 

 proboscis ; locomotion reptatory. 



Fig. 206 



Planaria maculata. 



This sub-order contains many more species than the 

 preceding, and to it belong the true Planarias. The 

 accompanying figure will represent the order Pla- 

 naria. — See fig. 20G. 



Order II.— TERETULARIA {The Nemertines). 



The worms of this order have a body linear in shape, 

 and very much (in some cases nearly a thousand times) 

 longer than broad. It is soft and glutinous, very often 

 covered with vibrating cilia, and so contractile, that 

 when extended in the act of moving through the water 

 or mud, they exceed their length when at rest by three 



