58 INVEKTEBRATE ANIBIALS. 



Class SCOLECIDA. 



(TKUiXYj^, a worm ; g'/'5oj, form. 



Unisegmental Worms (Rolleston). 



The head of the Tape-worm constitutes the true animal, the joints 

 being simply hermaphrodite generative products, which the head throws off 

 by a process of gemmation. 



Group 155. — PARASITIC SCOLECIDA; often termed 

 ENTOZOA. Tape-worms and Cystic- worms, Tceniada; 

 Flukes, Trematoda ; Thorn-headed Worms, Echino- 

 rhynchus ; Hair-worms, Gordiacea ; Thread-worms, 

 Ascaris, Oxyuris, Trichina; Guinea-worm, Filaria ; 

 &c. The members of this group may in a general 

 manner be described as passing through certain stages 

 of their development in the bodies of living animals. 

 The embryos when set free remain imperfectly developed 

 until received into some living animal other than the 

 one in which they began their existence. 

 H Illustrative drawings and diagrams. 



Group 156.— FREE SCOLECIDA. The free, or non-para- 

 sitic, Scolecida include animals belonging to both the 

 two systematic divisions of the class. In the division 

 Platyelmia, or Flat-worms, are the Turhellaria, includ- 

 ing the Planarians, which in shape resemble minute 

 soles, and are found in pools and moist places ; also the 

 Nemertians, or Ribbon-worms, interesting as resembling 

 the Sea-urchins in a certain stage of their larval develop- 

 ment. In the division Nematelmia, or Thread-worms, 

 most of which are parasitic, are the Anguillulidce, 

 Vinegar-eels, Paste-eels, &c., numbering more than 200 

 species, chiefly remarkable for tenacity of life. On the 

 authority of Professor Rolleston, a place in this class is 

 assigned to Sagitta, a transparent worm-like marine 



