ON THE ANGUILLULIDE. 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 à 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 (Rhabditis), since the marine species Monhystera ambigua and M. 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 ** esophageal 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 striæ of the 
integument, as I have never once met with it in any species presenting well-marked trans- 
verse strie, 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 Mi hystera, Cyatholaimus, 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 afford 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 
N 2 
