INVERTEBRATA. 439 



generative organs disappear. Ultimately groups of ripe 

 segments or proglottides become detached and pass to the 

 exterior, and there the eggs are dispersed. If swallowed 

 accidentally by the first host the eggs develop into the 

 cysticercus. Cysticercus pisiformis in the abdomen of the 

 rabbit becomes Taenia serrata in the intestine of the 

 dog ; Coenurus cerebralis of the brain of the sheep becomes 

 Taenia coenurus in the dog ; Echinococcus is a proliferating 

 cysticercus in the liver of man, sheep, pig, etc., and 

 develops into Taenia echinococcus in the dog ; the larva of 

 Taenia solium, which occurs in man, is a cysticercus in 

 " measly " pork ; while the larva of Taenia saginata, 

 another human tape-worm, occurs in beef. 



PHYLUM ANNELIDA. 



Bilaterally symmetrical, metamerically segmented ani- 

 mals in which each segment primitively contains a large 

 coelomic cavity, a pair of gonads, a pair of nephridia, 

 and a pair of coelomic ducts which are functionally 

 generative ducts ; nervous system consisting of a pair 

 of cerebral ganglia connected by perioesophageal com- 

 missures to a double ventral nerve cord, with more or 

 less distinct ganglion enlargements in each segment; 

 usually a closed vascular system, including a dorsal and 

 ventral vessel. The vascular system and the metamerism 

 distinguish the Annelida from the Platyhelmia, while the 

 condition of the coelom and vascular system are very 

 different from that seen in the Arthropoda. They in- 

 clude two principal classes, namely, the Chaetopoda and 

 the Hirudinea or leeches, and also a number of smaller 

 groups which for the present may be considered as classes, 

 namely, the Archiannelida, the Echiuroidea, the Sipuncu- 

 loidea, and the Priapuloidea. The last three were formerly 

 united in the Class Gephyrea. 



CLASS: CHAETOPODA. 



The possession of chaetae alone does not suffice to define 

 the Chaetopoda, for chaetae are present in the Echiuroidea, 

 but tlje Chaetopoda have well marked metamerism with 



