80 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



5. Excretory Organs. Seventeen pairs of complicated neph- 

 ridia succeed one another from the second to the eighteenth 

 segment. Their internal ends lie within, but do not open into, 

 special sections of the body-cavity communicating with the ventral 

 sinus, and their external ends open by ventral pores. 



A nephridium taken from the middle of the series is an up- 

 wardly projecting f*|" sna P e d loop, the limbs of which are anterior 

 and posterior. The walls of the loop are traversed by a compli- 

 cated system of intracellular ducts. The anterior limb commences 

 with a blind cauliflower-shaped end which lies in the special 

 sinus, and then runs upwards to be succeeded by the posterior 

 limb which ends blindly below. A narrow vesicle-duct runs from 

 the anterior limb to a vesicle which opens by a short tube to the 

 exterior. The vesicle is muscular and lined by ciliated epithelium, 

 which also extends into the tube leading from it. As in earth- 

 worm, therefore, the nephridial cavity is mainly intracellular, but 

 its last section is intercellular. The former part is closely asso- 

 ciated with an elaborate network of capillary vessels. 



Nitrogenous waste has been detected in the vesicles. 



6. Reproductive Organs (Fig. 23). The unpaired ventral aper- 

 tures of the hermaphrodite system have already been described. 



The male organs consist of spermaries, ducts, and a protrusible 

 penis. 



There are nine pairs of spherical spermaries (testes) in the ventral 

 region of segments 8 to 16. The testes on each side communicate 

 by means of wavy vasa efferentia with a sinuous spermiduct (vas 

 deferens) which enlarges in the sixth segment into a convoluted 

 epididymis. From this a short duct runs to the base of the pro- 

 trusible penis which lies in the same segment. 



The female organs consist of ovaries, oviducts, and vagina, all 

 of which are placed in the seventh segment. 



The ovaries are two small thread-like structures enclosed in 

 rounded sacs, from which two narrow oviducts proceed. These 

 unite to form an unpaired convoluted tube which is continuous 

 with a muscular sac-like vagina. 



Numerous sperms with rounded heads and vibratile tails are 

 produced in the testes by the division of mother-sperm-cells, and 

 passing down the spermiducts are bound into cylindrical packets 

 (sperm-ropes or spermatophores) by a secretion of the epididymes. 

 The penis is a copulatory organ by which these packets are intro- 



