CHAPTER XL 



Class II. — Scolecida. 



Iisr the second class of the sub-kingdom Annuloida are in- 

 cluded a number of organisms which are, in many cases, very 

 unlike one another in external appearance, but which, never- 

 theless, agree in one or two structural points of importance. 

 The most important of these are the possession of a system 

 of water-vascular vessels, the absence of a vascular system, 

 and the possession of a nervous system composed of no more 

 than one or two nervous masses or ganglia. The points by 

 which the Seolecida are distinguished from the Echinodermaia 

 are, the absence of calcareous matter in the skin, the absence 

 of any traces of a radiate arrangement of their parts, especially 

 of the nervous system, the constant absence of any blood- 

 circulatory apparatus, and the course of their development. 

 The Seolecida (Gr, sJcolex, a worm) are often vermiform in 

 shape, but many of them exhibit no worm-like characters, 

 and one whole order is entirely microscopic. A great many 

 of the Seolecida are internal parasites in other animals, and 

 these are often collectively spoken of as JEntozoa (Gr. entos, 

 within ; zoon, an animal). These parasitic forms subsist by 

 an imbibition of the juices of their host through their 

 delicate integument. They have, therefore, no necessity for 

 acquiring food for themselves ; and we find, in consequence, 

 that many of them are wholly destitute of an alimentary 

 canal, and that in all the organs of " relation " are very rudi- 

 mentary. The Seolecida are divided into the following seven 

 groups or orders : 



1. Tceniada (Tape-wbrms). 



2. Trematoda (Flukes). 



3. Turbellaria (Ribbon-worms and Planarians). 



