49 



Subkingdom IV. VERMES. (Worms.) 



Body generally elongate or vermiform, soft, bilaterally sym- 

 metrical, with or without feet, but, when present, never jointed. 



There is a great diversity of form and structure in this sub- 

 kingdom, so that little can be said of it collectively. There is a 

 water-vascular system, and, occasionally, " pseudohaemal vessels 

 are present. A digestive cavity is sometimes absent. A few have 

 eyes, but sense-organs are of the simplest kind. Reproduction is 

 mostly by ova. Many species are external (Ectozoa) or internal 

 parasites (Entozoa). 



The water-vascular system is a tubular set of vessels having 

 openings on the surface of the body, and branching out into its 

 substance. It is never subservient to locomotion, as in the Echi- 

 nodermata. 



The classes composing this subkingdom are widely different 

 from one another, and were for a long time combined with 

 Arthropoda, under the name of Annulosa or of Articulata. 

 There are many points of approximation between them and the 

 Mollusca. 



A division of the classes has been made into Archgeostomata 

 (mouth the same through life) and Deuterostomata (mouth in 

 the adult a secondary development). The former includes Pla- 

 tyelmintha except Cestoda, Nematelmintha, Gephyrea, and Eoti- 

 fera; the latter Annelida except Hirudinea, Chzetognatha, and 

 Polyzoa. Giard unites Annelida, Polyzoa, and " satellite groups " 

 to the Mollusca, constituting his Gynotoca ; while Nematoda, 

 Gastrotrieha, &c. are his Nematelmia. 



Normally single animals. 

 Tail never fin-like. 



Body distinctly segmented. . . . ANNELIDA. 



Body not segmented, or very indistinctly. 

 No ciliated disk. 



Mouth simple or none. 



Body flat PLATYELMINTHA. 



Body rounded NEMATELMINTHA. 



Mouth at the end of a proboscis ... GEPHYREA. 

 rn ., c ate d disk at the anterior end ... ROTIFERA. 

 i ail nn -like p w ?Tnrivi 



-~, _ OH^ETOGN 



Compound plant-like organisms POLYZOA. 



