WORMS. 



■219 



Tlie nnnoli.ls Mttnin the higlicsl. ;nKit„.nicMl .j^r.uh- l<n.,un witliin Mr. vvnnian type. 

 By tliis I menu that their orgmis aiv nmre coniijlkutud, or as one says, specialized 

 This advance is recognizable in all parts; the sensory apparatus is nnu-h perfected 

 es]iecially the eye and ear have become capable of better fund ions than in lower 

 worms. The nervous system has distinct centres, or ganolia, of «],i,h tlic hn-est 

 lies in the head and is comjiarable with a true brain, and' exhibits, ui.on proper micro- 

 scopical examination, considerable complexity of structure. The i.eculiaritv, however, 

 which is most striking, and which gives the name to the class, is the division of tin' 

 body into a succession of more or less similar short parts, known as rings, joints, or 

 more correctly, as segments. In the common earth-worm these joints are i-eadily sLen^ 

 being marked on the outside. The essential part of a true segment is a distinct 

 division of the muscles, but besides we also find a separate nerve-centre for each seo-. 

 ment, a sej.arate excretoiy organ, sejiarate markings and se])arate external appenda^w 

 all repeated for each segment. In the grouj) known as the Polycha-ta, the extenial 

 ai)pendages are numerous and consjiicuous, and serve to accentuate, in the eyes of 

 even a hasty observer, the serial repetition of parts, whicli is the most obvious" irsult 

 of the segmentation of the body. 



The annelids fall naturally into three well marked sul)-classes, the Archiannclida 

 havn)g no bristles and no suckers; the Chsetopoda, having bristles upon their sides,' 

 and the leeches, having two suckers, one around the mouth and the other on the ven- 

 tral side of the j)osterior extremit}'. 



Sub-Clas8 I. — Akciiianxelida. 



This sub-class includes a small number of worms, whose affinities and importance 

 as the nearest living representatives of the archetype of the 

 segmented worms was fiivst pointed out by Hatsehek, to 

 whom science is also indebted for the recognition of the 

 group, which is oidy just beginning to find its way into 

 textbooks (1884). The best-known form is PoJijcjonHuK 

 a long, slender worm, without bristles or appendages nor 

 external joints, although its body is segmented. Its larva 

 was long familiar to naturalists as a free swimming pelagic 

 embryo, very minute indeed, and always designated as F'"- -io--/'"'.'/.';on/ras larva; 



T * 1 1 -nr , *" anus; 7j;, mouth; .*.-, stomach. 



-Loven s larva. Many an attempt was made to trace out 



the metamorjihoses of this larva, but always unsuccessfully until within a few years, 



wdien it was discovered to be the young of Pohjgordins. 



Sub-Class II. — Ch^topoda. 



These worms may be easily recognized by their jointed bodies, and by the presence 

 of^ two rows of sjiines on each side thereof. The places of insertion for the 

 spines are often protuberant, and sometimes project so far as to form false limbs — 

 (parapodia). We then have on each segment four of these outgrowths, each bearing 

 a smgle spine or a group of spines. When upon each ])rotnberance there is but a 

 single, or very few spines, the worm belongs to the Oligochaita, the lower order of 

 the sub-class; in the Oligocha^ta the protuberances are but slightlv if at all marked; 

 in the other and higher order, the Polychteta, there are usually true parapodia,' 



