VERMES. 265 



SUB-KINGDOM II VEBMES. 



Bilaterally symmetrical animals without a skeleton, having a longer 

 or shorter cylindrical or flattened body, which is either perfectly simple 

 or exhibits a larger or smaller number of flat or ring-shaped segments, 

 which thus give their possessors a certain resemblance to the Arthropod a. 

 Dorsal and ventral surface often closely resemble one another. External 

 appendages are in many cases completely wanting. When present, they 

 consist either of organs of attachment (suckers, chitinous hooks, &c.\ or, 

 as in the majority of the segmented or ringed worms, of tufts of bristles, 

 which are then distributed very regularly over the segments. Sometimes 

 branchiae are also found ; if they be not present, the respiration is simply 

 cutaneous. The habitat is in water or in damp localities; the motion 

 is in general slow. 



The animals now included under the designation Vermes form a 

 group much more restricted than was originally intended by Linne in 

 his famous " Systema Natime." The latter included under that name 

 all the invertebrate animals, with the exception of the so-called 

 Arthropods (Insecta, L.) that is to say, a host of animals of the most 

 heterogeneous types, which Cuvier first taught us to distinguish. 

 Worms now include only a small group of these animals, namely, those 

 which remain after the exclusion of the Protozoa, Radiata, and Mollusca. 



To fix the natural limits and characteristics of this division is a 

 very difficult problem of systematic zoology, especially difficult be- 

 cause the worms, even as now limited, represent no uniform group. 

 For our present purpose it is sufficient to describe them as above. 



In regard to internal organization, the worms exhibit almost greater 

 contrasts than in external structure, for there is no single organ which 

 is even essentially the same throughout. Not only is the body-cavity 

 frequently wanting, so that the intestine and the cortical layer, other- 

 wise separated by it, are united into a common mass, but in many 

 cases neither blood nor blood-vessels are present. There are even 

 a great many worms, and particularly among those which we are 

 about to consider, which are entirely destitute of an alimentary canal, 

 although this is by far the most constant of all animal organs, and 

 is hardly ever wanting in the other Metazoa, The disposition of 

 the muscular and nervous systems also exhibits numerous and im- 

 portant differences, which are partly co-ordinated with the external 

 structure of the body. The only organ constantly present is an 



