252 



THE INVERTEBRATA 



the contraction of these muscles (Fig. 187 B). The larval mouth re- 

 mains in the adult. After metamorphosis the animal sinks to the 

 bottom and begins its adult life. 



A B 



Fig. 187. Development of Polygordius. After Woltereck. A, Trochosphere 

 with rudiment of prostomium and trunk. B, Metamorphosing larva with the 

 prostomium and trunk brought close together by the contraction of the 

 longitudinal muscles and the umbrella of the trochosphere shrivelled and 

 about to be discarded. Three segments only of the trunk are shown, brn. 

 brain; e. eye; m.l. longitudinal musculature; m.L' part of the same which by 

 contraction brings the prostomium and trunk rudiments into contact; M. 

 mouth; nep. protonephridium with solenocytes; pre. prostomium; prt. pro- 

 totroch; mtr. metatroch; oe.c. oesophageal commissure; ten. tentacle. 



Order OLIGOCHAETA 



Chaetopoda, nearly all land and freshwater forms, with a compara- 

 tively small number of chaetae, not situated on parapodia, with pro- 

 stomium distinct but usually without appendages ; always hermaph- 

 rodite, the male and female gonads being few in number (one or 

 two pairs), situated in fixed segments of the anterior region, the male 

 always anterior to the female; with special genital ducts (coelomo- 

 ducts) opening by funnels into the coelom, spermathecae, and a 

 clitellum present at sexual maturity ; with reproduction by copulation 

 and cross-fertilization; eggs being laid in a cocoon, developing 

 directly without a larval stage. 



In addition the pharynx is not eversible and pharyngeal teeth (such 

 as frequently occur in the Polychaeta) are absent, except in one small 

 family, the Branchiobdellidae, which have ectoparasitic habits similar 

 to the leeches and resemble them in some particulars of structure. 



