646 DB. H. CHAELTON BA8TIAN ON THE ANATOMY AND PHYSIOLOGY 



the influence of somewhat hasty and superficial views regarding the affinities of these 

 animals, leavened too powerfully by a consideration of their mere worm-like external 

 form. Moreover, the confusion has been heightened by the presence in the Nematoids 

 of rather anomalous structures — the so-called "lateral and median lines" — ^which, 

 though themselves neither vessels nor nerves, have, each in turn, been taken for both by 

 different observers. The intimate connexion and blending of the real nervous system 

 with these bodies in one part of their length, and its limited extent, account in a measure 

 for its having so long escaped detection, since this system cannot possibly be properly 

 examined without most careful dissection and after-preparation of the specimen. 



Having become accidentally interested in the anatomy of the Nematoids, and having 

 then been made fully aware of the unsatisfactory state of our knowledge concerning 

 them, I felt a strong desire to be able to remove some of the existing difficulties, so as 

 to place the subject upon a somewhat firmer foundation, and to discover what were 

 the real affinities of these animals. In addition to the present memoir, this desire has 

 resulted in the discovery and description* of 100 new species of free Nematoids, 

 partly marine and partly freshwater; and my researches on this branch of the 

 subject have convinced me that these may be considered to constitute one of the most 

 numerous and vndely distributed families in the whole animal kingdom, yielding in this 

 respect, perhaps, only to the ubiquitous Diatomaceae in the vegetable world. Their 

 bodies are so transparent, and enticing for microscopical examination, that I was tempted 

 on and on, far beyond the limits I had originally intended, in the hopes of being able 

 to learn something from them concerning the moot points in Nematoid anatomy. But 

 although many interesting facts have been thus acquired, still more have been gathered 

 from my later investigations into the structure of the parasitic Nematoids — though, 

 fortunately, the two sets of observations mutually throw light on one another. 



The recognition of the great numerical abundance and wide distribution of these free 

 Nematoids tends to throw much additional interest over the order Nematoidea, and 

 make the Nematoids as a group quite unique among other animals ; for in them we see 

 a great assemblage, one division of which has long been known to constitute a section 

 of the class Entozoa, most remarkable for the number of its representatives and the 

 frequency with which they are met in the most varied organs of animals belonging to 

 every grade from the Acalephse upwards ; whilst the other is now, also, known to be 

 composed of animals, in all probability infinitely more numerous still, leading a free 

 and independent life in all stages of their existence, tenanting almost every variety of 

 natural external habitat where moisture exists, and even invading in some cases, as para- 

 sites, representatives of the vegetable kingdom. Yet, strange to say, the organization of 

 these latter animals, as a whole, difiers in no very obvious or important manner from that 

 of their parasitic kindred. In accordance with their requirements, the sense-organs in 

 many of the free Nematoids become more numerous, and other modifications obtain ; still, 

 so far as we have yet been able to ascertain, their essential structure is not materially 

 * Monograph on the Anguillulidse or Free Nematoids, Trans, of Linn. Sec. vol. xxv. p. 72. 



