ON THE ANGUILLULID^. 91 



fusing the nomenclature, since I feel quite convinced, from an examination of the beau- 

 tiful figures he has given of these forms, that they cannot properly be included under 

 less than from eight to twelve distinct genera. 



So far as my own experience goes, I feel assured that even now, with the accession of 

 new forms brought to light by myself, it is altogether premature to attempt anything 

 like a philosophical classification ; we are as yet but on the threshold of our knowledge 

 of the multiplicity of types which will doubtless soon be revealed if the investigation 

 is taken up by naturalists at home, and a fortiori if the subject enlists the atten- 

 tion of scientific observers in various quarters of the globe. In this memoir I have 

 accordingly not ventured upon what may be called a classification, though I have care- 

 fully drawn up tables presenting a differential analysis of the characters of those of the 

 genera whose anatomical details are sufficiently known. This has been done principally 

 with the view of assisting in the identification of the species already described. On look- 

 ing over these tables, one cannot but be struck with the fact of the almost universal dis- 

 tinctness of the land and freshwater from the marine types. In only one undoubted 

 instance have I met with representatives of the same genus inhabiting both fresh and 

 salt water {Bhabditis), since the marine species Monhystera ambigua and 31. disjuncta, at 

 present placed in this freshwater genus, will in all probability ultimately be found to 

 belong to a distinct type, by virtue of certain anatomical peculiarities which distinguish 

 them from other species of that genus in which they have been temporarily placed. 

 One species of the freshwater genus Dorylaimus is also reported to have been found in 

 salt water by Dujardin. 



The ventral gland, or excretory organ, does not appear to be so common in the fresh- 

 water as in the marine genera ; and, as far as I have recognized it in the former, it presents 

 certain structural peculiarities. The peculiar " oesophageal ring," too, I have only met 

 with as unmistakeably existing in some of the marine genera, and in these, curiously 

 enough (though in this respect my experience appears to be contrary to that of Dr. 

 Eberth), only amongst such as have either longitudinal or no perceptible striae of the 

 integument, as I have never once met with it in any species presenting well-marked trans- 

 verse strife. The ocelli are much more marked and more frequent in the marine species, 

 though even the possession of such a well-marked appendage as this is not a character of 

 constant generic importance. In the genera Monhystera, Cyatholaimtis, and Chromadora, 

 for instance, certain species are provided with ocelli, whilst others are without them ; 

 and their presence or absence seems frequently to be connected with the nature of the 

 habitat. The degree of complexity of the male intromittent organs is also increased in 

 the marine genera, since in these as many as two or even four accessory pieces may 

 exist, whilst in the land and freshwater types the spicules are either solitary or provided 

 with one single, posterior, median accessory piece. The shape and number of these 

 organs aff'ord excellent generic characters of a most constant kind, with the exception 

 that occasionally, in genera whose species have spicules only, representatives will be met 

 with presenting also a single posterior accessory piece. Such is the case in the genera 

 Oncholaimus, Comesoma, and Monhystera. It may be, it is true, that this accessory 

 piece exists in a membranous and undeveloped condition in the other species, and so is not 



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