48 RAMBLES OF A NATURALIST. 



before backwards, each succeeding the last formed 

 one. We see, therefore, that the embryo will be an 

 articulate animal with its body divided into segments. 

 In this state it reminds us by its external form of 

 the leech, but cirrhi soon appear upon the sides. We 

 now ask whether the young animal is allied to the 

 earthworm or the Nais*, which exhibit distinct rings 



* Every one is familiar with the earthworm, which is known in 

 zoology as the Lumbricus. The Nais resembles it in many par- 

 ticulars, but instead of living exclusively in the humid earth, it lives 

 in the water, at one time making for itself a passage or tunnel 

 within the mud, and at another time attaching itself to the branches 

 of aquatic plants. "We will here mention a curious result connected 

 with those great questions of general harmony which fall so essen- 

 tially within the province of modern science. The Lumbricus and 

 the Nais had at all times been classed in the same group with 

 Marine Annelids. But these first two genera have both sexes 

 reunited in one individual, whilst my own observations, confirmed 

 by a great number of zoologists, have demonstrated that in the 

 Annelids the sexes are separate ; hence it is necessary to place 

 these animals in a different group. On the other hand, the resem- 

 blances which had caused them to be united with the Annelids are un- 

 deniable, and I have therefore been led to regard these resemblances 

 as being indications not of direct, but of collateral affinities, or analogies. 

 On examining the entire group of Worms from this point of view, we 

 find that they may be separated into two series which I have 

 designated as that of Les Vers Monoiques, and that of Les Vers 

 Dioiques, of which I have given a table in the first note in the 

 Appendix of Volume I. 



Here a very remarkable general fact presents itself to our notice. 

 All the Lumbricidse comprising Nais, inhabit fresh water, whilst 

 all the Annelids, whether they belong to the wandering or the 

 tubicolous groups, live in salt water. The former inhabit lakes, 

 rivers, and brooks, whilst the latter are only found in the sea. The 

 older authors have indeed spoken of Marine Naides and Marine 

 Lumbricidae, but in examining the species which are thus designated 

 I have always found that they were either true Annelids or special 

 genera allied to that class, like the Echiuri and the Polyophthalmia. 



