186 LOWER INVERTEBRATES. 



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

 habiting ponds and rivers and peopling the sea. The adults frequently exhibit a 

 marked preference for a living burial and inhume themselves in sand and 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 and number on sandy 

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

 sunken in the ooze the 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 Sagitta, are called pelagic, for they swim 

 about upon the ocean surface in company with the embryos and larvae 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 mentioned the parasitic life adopted 

 by a large number of the members of the group; 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 part of the body. In the fol- 

 lowing pages the habitats are considered with no little detail, so that we need not 

 occupy ourselves longer with the general subject. 



The worms fall naturally into a number of distinct classes, some of which com- 

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

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

 placed 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 majoritj' of 

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

 peers, notwithstanding the disparity of numbers between them, because the anatomy 

 of the Echinorhynchus entitles it to as distinct a rank as is given to the annelids col- 

 lectively. The reader therefore must not wonder at tlie inequality 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 conceriieil. 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 been simi- 

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

 the course of their metamorphoses 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 tyjies known as the Plathel- 

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

 simpler than the Trochozoon ; the doubt as to the aftinities 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 simply organized small 

 group, the Gastrotricha, by way of which we pass off from the main line of progress 

 to the nematodes, to Gordius and Mermis. The parasitic Echmorhynchi arc 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 

 Sagitta, and the cognate Chcetosoma and Desmoscolex. Returning now to the main 

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



