CHAPTER XI 
PHYLUM ANNELIDA 
Chief Classes—CuH&TOPODA, DISCOPHORA 
THe Annelids or Annulata are segmented “worms,” in 
most of which the segmentation of the body ts visible exter- 
nally. The head usually consists of a pre-oral “ prostomium” 
and a post-oral peristomium. The body wall has several 
layers. of muscles, and many, eg. Chetopods, have sete 
embedded in the skin. In most, there is a well-developed 
calom, communicating with the exterior by patred nephridia. 
The nervous system consists typically of two dorsal cerebral 
ganglia, a commissural ring round the gullet, and a ventral 
ganglionated chain. The gonads arise on the calomic ept- 
thelium. Not infrequently the nephridia function also as 
genital ducts. The development may be direct or indirect, 
and if indirect it usually includes a larval Trochosphere stage. 
In habit, form, and structure the Annelids exhibit much 
diversity. The Chztopods, represented on the one hand 
by the familiar earthworm, and on the other by the marine 
worms, best illustrate the typical Annelid structure. With 
these, however, may be included the aberrant Echiuride, 
e.g. Echiurus and Bonellia. A few primitive forms (Archi- 
Annelida), and the Myzostomata (parasitic on Crinoids), 
may also be appended to the Cheetopod class. The leeches 
(Discophora) are probably Annelids which have diverged 
in consequence of a peculiar half-parasitic habit. Finally, 
some zoologists include Sagit#fa (Chzetognatha) in this series 
as an Annelid with three segments, and also the Rotifers 
(Rotatoria), whose adult form somewhat resembles the 
‘Trochosphere larvee of many Annelids. 
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