ANNULOSA. 563 



their peculiar functions, they singularly resemble true 

 water-vessels ; they open by an external pore, and are 

 ciliated internally ; they unite around the gullet, as do the 

 water- vessels in some Trematoda, and are eventually shut 

 off from such communication; the ambulacral vessels of 

 the Holothuridce undergo precisely this change, and thus 

 they facilitate our comprehension of a transition from the 

 water-vessels of the Trematoda to the pseud-hsemal vessels 

 of the Annelida. We may take it as an established fact 

 that, whatever the functions of this varied vascular system 

 and its contents in different classes of the Annuloida, 

 they have nothing to do with the blood or true blood 

 vessels. The latter are entirely absent in all the Annu- 

 loida at present known, the blood (improperly called 

 " chyle-aqueous fluid ") being simply contained in the peri- 

 visceral cavity and its processes. The development of 

 the Nematoidea appears to take place without metamor- 

 phosis ; the embryo assuming within the egg a form nearly 

 resembling that of the adult. Encysted asexual nematoid 

 worms are frequently found in various parts of the body 

 of fishes; and the remarkable Trichina spiralis is the 

 asexual state of a nematoid worm, encysted within the 

 substance of the muscles of man. Zooid development is 

 only known to occur in one nematoid, the Filaria, Medi- 

 nensis, Guinea-worm. Mr. Busk's careful observations 

 have long since placed this fact beyond a doubt. 



The TceniadcB and the Acanthocephala, like the Trema- 

 toda, entirely parasitic in their habits, differ from them in 

 the total absence of mouth or digestive cavity. The 

 Taniadce, Tapeworms, are ribbon-like creatures, usually 

 divided throughout the greater part of their length into 

 segments, whose usual habitation is the intestinal cavity 

 of vertebrate animals ; and apparently of a carnivorous 

 vertebrate, in fact, though capable of existence elsewhere, 

 it is there alone that they are able to attain their complete 

 development. The anterior extremity of a taenoid worm 

 is usually called the head, and bears the organ by which 

 the animal attaches itself to the mucous membrane of the 

 creature which it infests. These organs are either suckers 

 or hooks, or both conjoined. In Tcenia, four suckers are 

 iombined with a circlet of hooks, disposed around a median 

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