Chap. I. THEIR SENSES. 19 



which runs in a straight course to the vent 

 at the posterior end of the body. The intes- 

 tine presents a remarkable structure, tlje 

 typhosohs, or, as the old anatomists called it, 

 an -intestine within an intestine ; and Clapa- 

 rede* has shown that this consists of a 

 deep longitudinal involution of the walls of 

 the intestine, by which means an extensive 

 absorbent surface is gained. 



The circulatory system is well developed. 

 Worms breathe by their skin, as they do not 

 possess any special respiratory organs. The 

 two sexes are united in the same individual, but 

 two individuals pair together. The nervous 

 system is fairly well developed ; and the two 

 almost confluent cerebral ganglia are situated 

 very near to the anterior end of the body. 



Senses. — Worms are destitute of eyes, and 

 at first I thought that they were quite in- 

 sensible to light ; for those kept in confine- 

 ment were repeatedly observed by the aid of 

 a candle, and others out of doors by the aid 

 of a lantern, yet they were rarely alarmed, 

 although extremely timid animals. Other 



* Histolog. Untersucliungen iiber die Eegenwiirmer. * Zeit- 

 schrift fiir wissenschaft. Zoologie,' B. xix., 1869, p. 611. 



