WORMS. 



189 





Fia.ie5. — Poly- 

 celis ioRvigata; 

 a, eyes. 



tube; there is a main central stem running through the front half of the body, and 



giving off many lateral ramifications ; at the middle of the body the main stem forks, 



producing two branches, which run backwards, and give off many secondary branches. 



Where the three main stems unite, springs off the long, muscular proboscis, which is 



an extensive cylinder for seizing and swallowing food. The proboscis will swallow 

 everything small enough ; in fact, its deglutive propensities persist even 

 a after the death of the worm ; for, sometimes, when anatomical research 



has destroyed all the tissues of the creature, the proboscis, being much 

 tougher than the rest, still remains intact, and goes on swallowing every- 

 thing it can seize, as if frenzied by hunger. 



There are many planarians having a branched digestive tract, and 

 they are all classed together, under the common appellation of Dendro- 

 coela, in opposition to the remaining Turbellaria, which have a simple 

 straight tract, and are therefore called Rhabdocoela. It has been as- 

 serted by some writers that there are other species with no digestive 

 canal, for which the term Acoela has been proposed. It seems, how- 

 ever, more probable that the authors alluded to have been careless in 

 their observations and hasty in their conclusions than that there are 

 any planarians really lacking a digestive tract. 



Of the Dendroccela, one of the commonest is Polycelis Icevigata, of 

 which we give a good figure, borrowed from Dr. Schmidt's great work 



on the natural history = —~ -^^ _^,.^_^_ ^^^^ ^ ^ 



of the lower animals. ^ - ^ ^ 



Of the Dendrocoela, besides the F ^^^ — = — 



three species already mentioned, the 



naturalist distinguishes many others. 



A few are found in moist earth, a 



goodly number in fresh water, but the 



majority are marine. The marine 



forms differ, for the most part con- 

 siderably, from the genera above de- 

 scribed. The reproductive organs, 



which are quite complicated in all 



Plathelminths, are especially different. 



The body is generally very broad and 



thin, often translucent and beautifully 



colored. In some cases a large size, a 



length of several inches, is attained. 



By way of illustration, we can men- 

 tion only a single very beautiful form, 



very common in the Bay of Naples. 



The animal ( Thysanozoon) may grow 



to nearly an inch in length. The back 



is covered with many rows of dark 



colored papillae. On the head end is 



a pair of ear-shaped outgrowths, which 



project obliquely upwards, and appear to act as tentacles, with a most sensitive touch. 



The ventral surface is pure white. The artist has represented the animal clinging 



Fig. 166. — 'I'hysanozoon. 



