38 WORMS. 



ganglion, are two nerve-strands : these in each segment of the 

 worm swell out into two ganglia, which are united by connect- 

 ing nervous matter called commissures. This chain of ganglia 

 and the nerves attached form the ventral nerve-cord. 



Eyes, auditory organs, and tactile organs are sometimes united 

 to the nervous system, forming primitive sense organs. 



A vascular sijdem is present in some groups, such as the 

 Ohii'toj/oda, Kemertinea, and Hirudi)iea. The blood is gener- 

 ally yellow or green, sometimes quite clear, and occasionally 

 red. It is red in the earth-worm, in which the vascular or 

 blood system is best studied. It consists of dorsal and ventral 

 blood-vessels united by lateral vessels, and it is also provided 

 with paired swollen trunks called " pulsating " or " blood 

 hearts." Worms respire through the whole body surface, 

 except in some marine Ghcet02Joda, which have specially modi- 

 fied branched gills found at the base of the limbs. Ej-crefnri/ 

 (irtjam are also present, consisting of paired canals called 

 " nephridia,'' which may open by a single pore {yematoda). 

 In the segmented worms, like the earth-worm, these nephridia 

 are paired in each segment. The nephridia, which are the 

 equivalent of the kidneys in the higher animals, open into the 

 interior or body cavity of the worm by a ciliated funnel, and to 

 the exterior by a small pore on each side of the segment. 



Both sexual and asexual reproduction take place in worms — 

 asexually, by fission and gemmation, amongst the lower forms. 

 In many cases both male and female organs are found in the 

 same individual, when it is known as an hermapJirodife. This 

 we can readily observe in the earth-worm. JIany others (most 

 of the Nematoda) have the sexes in separate individuals, the 

 male being, as a rule, much smaller than the female. Both 

 direct and indirect development take place. !Many species pass 

 through a metamorphosis, the egg giving rise to a larval form, 

 which by a series of changes assumes the adult condition. In 

 the parasitic species two hosts are often requisite for develop- 

 ment, and in these parasitic forms the embryonic stage is some- 



