132 LECTURE XT. 



and such the progress each system of organs has made in the transit 

 from the Nematoneurous to the Homogangliate types. The Anellides 

 are distributed into orders, according to obvious and easily re- 

 cognisable modifications of the locomotive and respiratory organs ; 

 which characters fortunately coincide with the general conditions and 

 grades of their organisation, and are therefore natural ones. Dr. 

 Milne Edwards, the pupil of Cuvier who has devoted most attention 

 to the Vermes thus grouped together by his great master, divides 

 them into four orders. 



The first is the Anellata suctoria, and comprises the leeches, 

 which are provided with a suctorial disc at each extremity of the 

 bod}', and have neither bristles nor tuberculate feet. 



The second order is the Anellata terricola, which includes the 

 earth-worms ; these have neither tubercular feet, nor external gills, 

 nor suckers, but are provided with short stiff" bristles fulfilling the 

 function of feet. 



The third order is the Anellata tnhicola, and includes all those 

 which are provided with setigerous feet and have the respiratory 

 organs at the anterior extremity of the body. The Anellides of 

 this and the two preceding orders can scarcely be said to have a 

 distinct head. 



The highest organised Anellides are also the most locomotive : 

 they have been called Errantes by Dr. Edwards. In them, the 

 respiratory organs are most developed, and from their position, 

 Cuvier, who first defined the order, has denominated it Dorsi- 

 hranchiata, the gills being attached to the sides of the body on the 

 dorsal aspect, along the middle part, or through the whole length of 

 the body. They are provided with setigerous processes for loco- 

 motion, and have always a distinct head. They are commonly known 

 by the name of Sea-centipedes, Sea-mice, or Nereids, from the 

 Linnasan generic name Nereis, which is almost equivalent to the 

 present ordinal term, Errantes. 



The tubular sheaths and protractile bundles of bristles which 

 constitute the organs of locomotion in this order have been already 

 noticed in the general characters of the class. The integument is 

 naked, soft, vascular, and highly susceptible of impressions in all the 

 Anellides. It is sometimes red, sometimes the epidermis reflects 

 iridescent tints. In the tubicular order the habitations are commonly 

 formed of foreign substances, as particles of sand or shells agglutinated 

 together by the mucous secretions of the worm, which is sometimes 

 done v/ith a considerable degree of neatness and apparent skill, as in 

 the PectinaricB and Terebellce. The Serpula secretes a calcareous 

 tubular shell, consisting of carbonate of lime and animal matter, like 



