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, the 

 typhi osolis, or, as the old anatomists called 

 it, an intestine within an intestine ; and Cla- 

 parede* 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. Untersuchungen iiber die Regenwurmer. 'Zeit- 

 schrift fur wissenschaft. Zoologie,' B. xk., 1869, p. 611. 



c 2 



