12 A SYNOPSIS OF ANIMAL CLASSIFICATION. 



although in one group a reduplication of certain parts resembles a 

 definite segmentation. The nervous system consists of a central 

 ganglion at the anterior end, from which extend longitudinal cords 

 which in the case of the more attenuated forms may become a com- 

 plicated system with transverse commissures. Many are sessile or 

 possess organs for temporary attachment ; in the locomotive forms 

 motion is effected by contractions of a subcutaneous muscular layer, 

 or in a few primitive and minute forms, by a band of cilia, the 

 irochus. A large number of Vermes are parasitic. 



Class I. PLATYHELMINTHES Vermes flattened dorso-ventrally, 



and with the body-cavity entirely 

 filled with a tissue known as Paren- 

 chyma, necessitating in the larger 

 forms a dendritic branching of 

 alimentary and other systems. 

 Intestine incomplete (i.e., without 

 anal orifice). Hermaphroditic. 



Sub-class I. TnsBELLAEiA Free-swimming Platyhelminthes, 



of oval shape; body very contractile 

 and covered with cilia. 



Order i . Rhabdoccela Intestine a straight, unbranched 



tube. X Mesosiomum. 



Order 2. Triclada Intestine dendritic, with three main 



trunks — one anterior and two pos- 

 terior. X Planaria. 



Order 3. Polyclada Intestine like foregoing, but with 



more than two posterior branches. 

 X Planocera. 



Sub-class II. TKEMATODES Parasitic Platyhelminthes. Boyd 



not ciliated in the adult, but gen- 

 erally furnished with sucking-discs. 

 Intestine much as in Triclada. 



Order i. Monogenea Ecto-parasites upon gills, integur 



ment, bladder, etc., of aquatic ver- 

 tebrates. Sucking-discs, three o- 

 more. Development direct. 



X Polystomum. 



