308 VERMES. 



find in the large marine Chcetopoda filiform or branched gills, which 

 are usually appendages of the parapodia (fig. 246). A respiratory 

 function may also be attributed to the tentacles of the Gephyrea. 



The excretory organs are represented by the so-called water-vas- 

 cular system, which consists of canals of different sizes, symmetrically 

 arranged and filled with a watery fluid in which granules are sus- 

 pended ; they communicate with the exterior through one or more 

 openings. The canals begin either as small passages in the tissues of 

 the body, or free funnel-shaped openings in the body cavity. In 

 the last case, they may subserve other functions ; for example, they 

 may conduct the generative products out of the body cavity. In the 

 segmented Vermes they are paired, and are repeated in each segment 

 as nephridia or segmented organs (fig. 70). A different arrangement 

 is presented by the two lateral canals of the Nematoda, which lie in 

 the so-called Literal lines or areas, and open by a common excretory 

 pore in the region of the pharynx. 



In addition to sexual reproduction an asexual multiplication by 

 means of gemmation and fission (rarely by formation of germinal 

 cells) is widely distributed, especially among the lower forms. 

 This asexual reproduction is, however, frequently 7 confined to the 

 larvae, which differ from the sexually mature animal in form and 

 habitation, and play the part of an asexual generation in the cycle 

 of development. Almost all the Platijhelminthes and numerous 

 Annelida are hermaphrodite ; the NematJielminthes, the Gephyrea, and 

 Rotifera, and also the branchiate Annelids are of separate sexes. 

 Many Worms pass through a metamorphosis : the larvae are charac- 

 terised by the possession of a prseoral ring of cilia (Loven's larva), 

 or of several rings of cilia. In the Cestoda and Trematoda, which 

 possess in their embryonic stage the capability of asexual reproduc- 

 tion, the metamorphosis assumes the form of a more or less com- 

 plicated alternation of generations which is often characterised 

 by the difference in the habitat of the two successive stages 

 of development and by the alternation of a parasitic and free 

 life. 



The vital activity of the Worms is in general of a low order, 

 corresponding with their habitat. Many of them (Entozoa) live as 

 parasites in the interior of the organs of other animals, some as 

 ectoparasites on the external surface of the body, and feed on the 

 juices of their hosts. Others live free in damp earth, or in mud ; 

 others, and these are the most highly organized forms, inhabit fresh 

 and salt water. 



