228 LECTURE XI. 



Such are the general anatomical characters of the class Annulata, 

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

 from the Nematoneurous to the Homogangliate types. The anellids 

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

 nisable 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 Annulata suctoria, and comprises the leeches, 

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

 body, and have neither bristles nor tuberculate feet. 



The second order is the Annulata 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, and which, in fact, are the rudiments of such. 



The third order is the Annulata tubicola, and includes all those 

 which are provided with setigerous feet and have the respiratory 

 organs at the anterior extremity of the body, whence they have also 

 been called Cephalo-branchiata. The anellids of this and the two 

 preceding orders can scarcely be said to have a distinct head. 



The highest organised Aniiulata are also the most locomotive : 

 they have been called errantia by Dr. Edwards. In them, the re- 

 spiratory organs are most developed, and from their position, 

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

 branchiata, 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 

 Linntean generic name Nereis, which is almost equivalent to the 

 present ordinal term. 



The tubular sheaths and protractile bundles of bristles which 

 constitute the organs of locomotion in this order have been already 

 noticed in the genei'al characters of the class. The integument is 

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

 anellids. It consists of a delicate non-ciliating epidermis, and of a 

 moderately firm corium composed of obliquely crossing filaments. 

 In some earth-worms the skin is red, from the colour of its contained 

 or subjacent circulating fluids ; in leeches it is variegated by a layer 

 of pigment-cells; in most anellids the epidermis reflects iridescent 

 tints, and in such it is readily detached from the cutis ; but it closely 



