82 WORMS, ROTIFERS, LEECHES, POLYZOA 



ditellum, while a double row of minute bristles extend along the 

 under surface of the body. The reproductive organs are herma- 

 phrodite, there being two male ducts and two female ducts, open- 

 ing on the ventral surface of the fifteenth and fourteenth segments. 

 The body cavity is divided by cross-partitions into segments cor- 

 responding to the external rings ; while the food canal is divided 

 into several parts, comprising a muscular pharynx for grasping 

 leaves or stones, a gullet, a crop, a gizzard or mill for grinding up 

 the swallowed soil, and a long digestive part. A pair of small 

 kidney- tubes, or nephridia, are present in almost every segment, 

 and represent the excretory system. The fertilised ova are en- 

 closed in a cocoon, the wall of which is composed of a viscid secre- 

 tion from glands on the saddle or ditellum. This cocoon is depo- 

 sited in the earth, and the embryos develop within it into perfect 

 minute worms ere they make their escape. The Earth-worms 

 are very widely distributed, and some of the tropical forms attain 

 to a very considerable length and thickness. 



The Many-bristled Worms (Polychceta) are highly developed 

 marine worms, varying greatly in appearance and habit, and are 

 divided into two sub-orders, namely, those leading a wander- 

 ing life and having well-developed false-feet or parapodia, 

 which comprise the Errantia, and those having feebly deve- 

 loped parapodia and dwelling in protective tubes, which comprise 

 the Tubicola. 



" In examining these Many-bristled Worms it is advisable to 

 employ certain descriptive terms. Thus, the first segment of 

 the body is called the prostomium, and the mouth opens on it ; 

 the second is the peristomium. When the three front segments 

 are united, or when they differ from those which come after, they 

 are called the head or cephalic segments ; but when this is not 

 the case the worm is said to be acephalous. The head has vari- 

 ous appendages, according to the genera. Antennae are soft fila- 

 ments varying in number from one to five, and they arise directly 

 from the head, are not retractile, and are usually jointed at the 

 base. The tentacles are soft, bristly, or thread-shaped, non- 

 retractile processes, which arise from each side of the segments of 

 the head in pairs, and spread laterally ; they are often very long, 

 and are contractile in the acephalous genera. The mouth is under- 

 neath the head, and is a round or transverse opening to the gullet. 



