158 MANUAL OF ZOOLOGY. 



living in being composed of not more than twenty rows of 

 calcareous plates. The Oolitic and Cretaceous Rocks are 

 especially rich in forms belonging to this order, many genera 

 being peculiar ; but the number of forms is too great to permit 

 of any selection. 



7. HOLOTHUROIDEA. This order, comprising, as it does, soft- 

 bodied animals, can hardly be said to be known as occurring 

 in the fossil condition. Some calcareous plates and spicules, 

 supposed to belong to a ffolothnrid, have, however, been de- 

 scribed as occurring in the Secondary Rocks, and the shield 

 of Psolus has been found in Post-tertiary deposits in Bute. 



CHAPTER XXIV. 



SCOLECIDA. 



CLASS II. SCOLECIDA. This class was proposed by Professor 

 Huxley for the reception of the remaining members of the 

 Annuloida, comprising the Rotifera, the Turbellaria, the Trema- 

 toda, the Tornado^ the Nematoidea, the Acanthocephala, and the 

 Gordiacea. Of these the Rotifera stand alone, whilst the Tur- 

 bellaria, Trematoda, and Tczniada constitute the old division of 

 the Platyelmia (Flat Worms), and the Nematoidea, Acantho- 

 cephala, and Gordiacea make up the old Nematelmia (Round 

 Worms or Thread-worms). For some purposes these old divi- 

 sions are sufficiently convenient to be retained, though they 

 are of little scientific value. The term Entozoa has acquired 

 such a general currency, that it is necessarily employed occa- 

 sionally, but it has been used in such widely different senses 

 by different writers, that it would be almost better to discard 

 it altogether. It certainly cannot be used as synonymous with 

 Scolectda, many of these not being parasitic at all. It will, 

 therefore, be employed here, in a restricted sense, to designate 

 those orders of the Scolecida which are internal parasites, 

 comprising the Trematoda, Ttzniada, Nematoidea (in part), 

 Acanthocephala, and Gordiacea. The Turbellaria and Rotifera, 

 with a section of the Nematoidea, lead a free existence, and are 

 not parasitic within other animals. 



The Scolecida are denned by the possession of a "water- 

 vascular system," consisting of a "remarkable set of vessels 

 which communicate with the exterior by one or more aper- 

 tures situated upon the surface of the body, and branch out, 



