WORMS. 219 



Tlie annelids attain the highest anatomical grade known within the vertiiian type. 

 By this I mean that their organs are more complicated, or as one says, specialized. 

 This advance is recognizable in all parts ; the sensoiy apparatus is much perfected, 

 especially the eye and ear have become capable of better functions than in lower 

 worms. The nervous system has distinct centres, or ganglia, of which the largest 

 lies in the head and is comparable with a true brain, and exhibits, upon proper micro- 

 scopical examination, considerable complexity of structure. The peculiarity, however, 

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

 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 readily seen, 

 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 seg- 

 ment, a separate excretory organ, separate markings and separate external appendages, 

 all repeated for each segment. In the group known as the Polychfeta, the external 

 appendages are numerous and conspicuous, and serve to accentuate, in the eyes of 

 even a hasty observer, the serial repetition of parts, which is the most obvious result 

 of the segmentation of the body. 



The annelids fall naturally into three well marked sub-classes, the Archiannelida, 

 having no bristles and no suckers ; the Chsstopoda, 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 posterior extriemity. 



Sub-Class I. — AncHiAisnsrELiDA. 



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 first pointed out by Hatschek, to 

 whom science is also indebted for the recognition of^the 

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

 text books (1884). The best-known form is Polygordius, 

 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 al\vays designated as ^'^n4'°.T:SfhTtstonS."' 

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



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

 when it was discovered to be the young of Polygordius. 



Sub-Class II. — Ch^topoda. 



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

 of two rows of spines 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 single spine or a group of spines. "When upon each protuberance there is but a 

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

 the sub-class ; in the Oligochseta the protuberances are but slightly if at all marked ; 

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



