ANNULOSA: ANNELIDA. 



259 



In the higher Annelida, the segmental organs are usually in 

 part subordinated to the function of reproduction. In these 

 cases (fig. 127) the inner surface of the convoluted tube, which 

 constitutes the segmental organ, is cili- 

 ated ; and the tube not only opens ex- 

 teriorly by a distinct " stigma," but also 

 communicates internally with the peri- 

 visceral cavity by a widely patulous, 

 trumpet - shaped, internally-ciliated in- 

 fundibulum (fig. 127, i\ by which the 

 products of generation are taken up and 

 conveyed to the outer medium. Very 

 usually, also, there are appended to the 

 tube of the segmental organ blind glan- 

 dular pouches, which represent the 

 kidneys, or in other cases caecal appen- 

 dices (fig. 127, s) for storing up the gen- 

 erative products. 



The nervous system consists of a 

 double, ventral, gangliated cord, which 

 is traversed anteriorly by the oesopha- 

 gus; the " prse-cesophageal," or "cere- 

 bral," ganglia being connected by lateral 

 cords or commissures with the " post- 

 cesophageal" ganglia. Pigment-spots, 

 or "ocelli," sometimes of high organ- 

 isation, are present in many, generally 

 upon the proboscis, sometimes in each 

 segment, or on the branchiae, or on the 

 tail; and the head often supports two 

 or more feelers, which differ from the 

 <; antennae " of Insects and Crustacea in not being jointed. 



The sexes in the Annelida are sometimes distinct, and some- 

 times united in the same individual. The embryos are almost 

 universally ciliated, and even in the adult cilia are almost 

 always, if not always, present in both of which respects this 

 class differs from the Arthropoda. 



The Annelida may be divided into two sections, characterised 

 by the presence or absence of external respiratory organs or 

 branchiae. The Abranchiate section comprises the Leeches 

 and the Earth-worms ; whilst the Branchiate division includes 

 the Tube-worms (Tubicola) and the Sand-worms (Errantia). 

 The Annelida are also often divided into two sections, called 

 Chcetophora and Discophora, according as locomotion is effected 

 by chitinous setae (Earth-worms, Tube- worms, and Sand-worms) 

 or by suctorial discs (Leeches). 



Fig. 127. Segmental organ of 

 of a Chsetopodous Annelida 

 (Alciopa), enlarged, o Ex- 

 ternal aperture or stigma ; t 

 Tubular and ciliated portion 

 of the segmental organ ; s 

 Seminal receptacle; z Cili- 

 ated infundibulum opening 

 into the body-cavity. (After 

 Claparede.) 



