ANNULOSA : SCOLECIDA. 229 



THE SCOLECIDA. 



The name of Scolecida was proposed by Professor Huxley * 

 for the reception of the Rotifera, the Turbellaria, the Trema- 

 toda, 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 ac- 

 quired 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, Taniada, Nematoidea (in 

 part), Acanthocephala, and Gordiacea. The Turbellaria and 

 Rotifera, with a section of the Nematoidea, lead a free exist- 

 ence, and are not parasitic within other animals. 



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

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

 which communicate with the exterior by one or more apertures 

 situated upon the surface of the body, and branch out, more or 

 less extensively, into its substance" (Huxley). No proper vas- 

 cular apparatus is present, and the nervous system (when pre- 

 sent) "consists of one or two closely approximated ganglia." 

 The body is not segmented, or but imperfectly so, and lateral 

 appendages are absent in all except certain of the Rotifers. 

 The habits and mode of life of the different members of the 

 Scolecida are so different, that no other character, save the 

 above, can be predicated which would be common to the 

 entire group, and would not be shared by some other allied 

 division. The most important morphological feature by which 

 the Scolecida are separated from the Annelida, is that they are 

 destitute of the ventral gangliated nerve- chain which is so 

 characteristic of the latter group. 



* More recently ('A Manual of the Anatomy of Invertebrated Animals,' 

 1877) Professor Huxley has abandoned the division of the Scolecida, and 

 has separated its members into two sections (Trichoscolices and Nemato* 

 sco lie es). 



