ANNULOIDA : SCOLECIDA. 183 



comprise all the known forms of the family, the former occur- 

 ring in the Devonian Rocks, the Carboniferous Limestone, and 

 the Permians, whilst the latter occurs also in the Upper 

 Silurians. 



The Secondary and Tertiary Echinoidea resemble those now 

 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 Holothurid, have, however, 

 been described 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 

 Trematoda, the Tceniada, the Nematoidea, the Acanthocephala, 

 and the Gordiacea. Of these the Rotifera stand alone, whilst 

 the Turbellaria, Trematoda, and Tceniada constitute the old 

 division of the Platyelmia (Flat Worms), and the Nematoidea, 

 Acanthocephala, and Gordiacea make up the old Nematelmia 

 (Round Worms or Thread-worms). For some purposes these 

 old divisions 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 

 occasionally, 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 synony- 

 mous with Scolecida, 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 



