186 LOWER INVERTEBRATES. 



All worms appear to require moisture, aud the majority of them are aquatic, in- 

 habiting ponds and rivers aud ])eo]jling the sea. The adults frequently exhibit a 

 marked preference for a living burial and inhume themselves in sand aud mud, some 

 at the bottom of stagnant waters or running streams, others in the floor of the ocean 

 at all dejiths ; but they are found in the greatest variety aud number on sandy 

 beaches, which the changing tides alternately cover and expose. Under stones or 

 sunken in the ooze tlie collector gathers them in astonishing abundance. In the moist 

 earth, especially in vegetable humus, and in manured soil live the common earth 

 worms. Some species, like those of the genus iSagittu, are called ])elagic, for they swim 

 about upon the ocean surface in company with the embryos and larvue of worms of 

 many kinds and a marvellous society of other living things. Rotifers and others 

 swim in fresh water as well. Finally is to be mciitioued the pai-asitic life adojited 

 by a large number of the members of the grouji; in the infested hosts they find all the 

 necessary conditions for their existence. Such parasites are more common in the in- 

 testine than in any other organ, but they attack every ]iart of the body. In the fol- 

 lowing pages the habitats are considered witii no little (Ictail, sn that we need nut 

 occu])y ourselves longer with the general subject. 



The worms fall luiturally into a number of distinct classes, some of ^\•hieh com- 

 prise but a single genus, while others are large groups and present a multitude of 

 forms. The Echinorhynchus is so isolated among living worms that usually it is 

 ])laced from anatomical reasons by itself ; the jointed worms or annelids, on the con- 

 trary, have more representations in the earth's present fauna than the majority of 

 classes among animals. Yet these two classes are regarded by most zoologists as 

 peers, notwithstanding the dis])arity of numbers between them, because the anatomy 

 of the Echinorhi/nchus entitles it to as distinct a rank as is given to the annelids col- 

 lectively. The reader therefore must not wonder at the inequalitj' in size of the 

 co-ordinate divisions of worms. 



The classification here adopted is that which appears to me to best accord with 

 our present knowledge of the animals concerned. All the forms are bound together 

 by a hypothetical link, the Trochozoon, which is also the starting point of the Mol- 

 lusca and all bilaterally symmetrical animals. This Trochozoon must have l)cen simi- 

 lar in organization to those little creatures, the wheel animalcules or Rotifera, and in 

 the course of their metamori)hoses the young of many worms and annelids pass 

 through what is known as the Trochozoon or Trochophora stage, so that the life his- 

 tory of the individual proves that the adult is derived by modification of the Trocho- 

 zoon type ; hence the induction is probable that the ancestors of these animals were 

 Trochozoa. This necessitates placing the hypothetical animal at the base of the 

 system. It is, however, still questionable whether the low types known as the Plathel- 

 minthia, that is the taj)e-worms and their allies, are not derived from something still 

 shupler than the Trochozoon ; the doubt as to the affinities of this class renders it 

 desirable to isolate it somewhat, as is done in the adjoining diagram. The rotifers 

 come very near the ancestral forms. Next we place a very simjih' organized small 

 grou)), the Gastrotricha, bj- way of which we pass oft' from the main line of progress 

 to the nematodes, to Gordius and Mermis. The parasitic Echinorlii/nchi are usually 

 associated with the nematodes rather than with any other group by systematists, but 

 their true affinities are by no means definitely settled. Keeping on we come to the 

 Saffiita, and the cognate Chcftosoma and Desmoscole.v. Returning now to the main 

 line we approach the annelidan type. Here we must put first the nemertean or pro- 



