WORMS. 186 



Branch V. — VERMES. 



The great and varied assemblage of animals which are jDut together under the 

 coriimon designation of worms does not present a homogeneous group for study. On 

 the contrary many distinct types have been thrown together to make the branch of 

 worais. Indeed it has been a long-standing current joke among zoologists that this 

 part of the zoological system was the garret, or as the German has it the Rtimpelkam- 

 mer, into which everything was carelessly thrown that did not properly belong else- 

 where, and had been therefore rejected from the other portions of the system of classi- 

 fication. The worms have thus come to be a collection of forms whose outfeide affini- 

 ties extend to nearly all other animals, while among themselves they fall into classes 

 not closely related with one another. These classes shade off in some cases towai-ds 

 other branches ; thus the rotifers ajDproach in their organization the molluscan type, 

 while the Annelida proper show in some respects unmistakable similarity with the 

 insects. Other classes, like the Acanthocej^hali and Enteropneusti {Balanoglossus) 

 attain an anatomical configuration which gives them a certain independence, a place 

 apart, in the zoological system. In brief, as the limits of the branch of woi-ms are 

 vague, and its components multifarious, therefore it is difficult to define the worms 

 with an accuracy corresponding to the requirements of a rigorous science. Tlie fol- 

 lowing definition is the most satisfactory I am able to give : — 



A worm is a bilaterally symmetrical animal, with a distinct head characterized by 

 the presence of the principal nervous centre or so-called brain. It is distinguished 

 from molluscs by the absence of a shell and of that modification of the skin, named 

 the shell gland, which forms the shell and is present at least in a rudimentary condi- 

 tion in all true molluscs. It is distinguished from Crustacea and insects by the want 

 of jointed limbs, and finally from the tunicates and vertebrates by the lack of a struc- 

 tural axis, the so-called notochord or chorda dorsalis, which gives the name of Chor- 

 data to the divisions last mentioned. As far as at present known no worm has a true 

 liver, a calcified internal skeleton, an organ homologous with the endostyle of ascidians 

 and thyroid gland of vertebrates, any tracheal tubes like those performing respiration 

 in insects, or finally any unicellular hairs. In fact a worm must be recognized as such 

 rather by the process of exclusion than by the observation of positive characteristics. 



To the scientific zoologist the worms are most interesting subjects of study, not 

 only from their manifold variety and strange life histories, but also from their relation- 

 ship with the higher types, the ancestral forms of which are with good reason sup- 

 posed to be more nearly represented by certain worms than by any other animals now 

 existent. The mind links in imagination these obscure and humble creatures with the 

 most exalted organisms, and finds in the secrets of their low organization the key to 

 the complex structure of the higher animals. We, however, shall not enter upon 

 these difficult discussions, where debate is still active, and the final decision uncertain. 

 Instead we shall be sufficiently occupied with studying the principal and most in- 

 teresting forms of vermian life as to their appearances and habits. Although a worm 

 is by popular fancy a loathsome thing, yet only some of them deserve opprobrium, 

 while many others are objects of great beauty, and others again are quaint; a few are 

 of great utility to man, and yet others are among man's most dreaded enemies. 



