PREDA TOR Y ANNELIDS- LEE CHES. 67 5 



habit of projecting the hinder part of its body from its burrow and waving it 

 incessantly in the water. But one of the best known of these aquatic worms 

 is the species called Nais proboscidea, which has long hair-like bristles jutting 

 out from the sides of its body, and a proboscis-like feeler projecting from 

 the forepart of its head. This worm may be regarded as more highly 

 organised than the earth-worm, as is indicated by the presence of a couple 

 of eyes upon its head. Like some of the Polychaata, too, it possesses the 

 power of reproducing by fission. Between two of the segments in the 

 posterior half of the body a mass of tissue appears, and from the front part 

 of this a new tail is formed for the original piece bearing the head, while 

 from its hinder part a new head develops for the part that was primarily 

 the tail-end of the parent form. It not infrequently happens that several 

 new worms start almost contemporaneously in this way one behind the 

 other, and before the severance between them is effected, they appear as 

 a chain of individuals, exactly as described above in the case of Autdytus, 

 one of the Polychceta. 



In addition to the Polychaeta and Oligochseta a third group, called Archi- 

 annelida, is now referred to the Chaetopoda, in spite of the fact that the 

 integument bears no bristles, and the only external signs of segmentation 

 are shallow grooves in the skin and serially arranged rows of cilia (fine hairs). 

 The head is furnished with tentacles, but there are no parapodia. This 

 group, lying in some respects midway between the Chaetopoda and the Flat- 

 worms of the sub-class Turbellaria, contains several marine forms, of which 

 the best known are Protodrilus and Polygordius. The young of the Archi- 

 annelida are developed from Trochophore larvae of the type known as 

 Telotrochous (see p. 673). 



Usually now regarded as aberrant members of the class Chaetopoda are 

 the remarkable species belonging to the genus Myzostoma, of the family 

 Myzostomidse. These degraded creatures live parasitically upon the Sea- 

 lilies (Crinoidea), their presence giving rise to tumours or various irregularities 

 of growth. The body is oval and flattened and unsegmented, and its margins 

 are produced into a number of short, prehensile filaments ; while on the 

 lower surface near the edge are four pairs of suckers, and on the inner side 

 of these five pairs of conical, foot-like processes, each of which is provided 

 with a pair of retractile bristles. But in spite of the anomalous structure 

 of the adult, these parasites are developed from free-swimming larvae 

 furnished with ciliated bands, and calling to mind the Trcchophore of the 

 Annelida. 



THE LEECHES SUB-CLASS HIRUDINEA. The Leeches, or Hirudinea (Latin, 

 hirudo, a leech), sometimes called the Discophora (disc-bearers), on account 

 of the suckers with which they are furnished, are not unlike earth-worms in 

 general appearance, and also in many points of structure. The integument 

 is divided by shallow transverse grooves into a multitude of rings, a varying 

 number of which (five to three) correspond to each true segment of the body. 

 There are no bristles nor organs of locomotion in the form of parapodia, 

 movement from one spot to another being effected by means of swimming 

 or by crawling, with the aid of the suckers, after the manner of the " looper '' 

 caterpillars. There is always a large sucker situated at the hinder end of 

 the body, and usually a second at the opposite extremity, with the mouth 

 placed in its centre. 



The Leeches are divisible into two families the Gnathobdellidce., or jaw- 



