128 HELMINTHOZOA. 



the ovary, which is connected with the long (ovigerous ?) filament above 

 alluded to. The nucleus is contained in a proper cavity, generally 

 distinct from the large excavation that forms the locomotive apparatus, 

 and is connected by filaments, apparently of a vascular character, to the 

 soft parts within the body. It has been already remarked that this 

 latter division is excavated by a large cavity that extends nearly through- 

 out its entire length ; from the bottom of this cavity arises a prolonga- 

 tion, probably of a vascular character, which embraces the root of the 

 (ovigerous ?) filament, and is apparently connected with the nucleus, 

 from which, however, it may be detached by the slightest effort. 



(333.) The bodies of these strangely -constructed creatures are so 

 extremely transparent, that their presence is discoverable with great 

 difficulty even in small quantities of sea-water. They are generally 

 met with at a great distance from land, abounding more especially in 

 the seas of tropical climates. They swim with great facility, their an- 

 terior or nuclear extremity being directed foremost ; while the water 

 taken into their bodies, being forcibly ejected, by the contractions of 

 their subcartilaginous parietes, through the wide apertures opening 

 backwards, propels them through their native element. 



(334.) Whilst exercising this mode of locomotion, the long slender 

 filament above alluded to is extended behind, being partially lodged in 

 a groove excavated in the posterior division of the natatory organ. It 

 varies considerably in length, being highly contractile, so much so, in- 

 deed, that it is sometimes completely withdrawn into the body ; and its 

 structure is further remarkable from the circumstance that through- 

 out its whole length it is furnished at regular intervals with minute 

 suckers*. But the true nature of this organ is very imperfectly known ; 

 most probably it will be found to be analogous in its real character to the 

 proligerous apparatus of the Salpce, to be described hereafter ; indeed, 

 such is the evident relationship between the Diphyea and the Salpoid 

 Tunicata, that it is very doubtful whether they ought not to be classed 

 as members of that group. 



CHAPTER VII. 



HELMINTHOZOA. 



(335.) THE HELMINTHOZOA, embracing the vast class of parasitic 

 worms, may be conveniently divided into two groups. First, those 

 which live as parasites the EISTTOZOA, and secondly, those which are 

 free and have an independent existence, as is the case with many of the 

 TREMATODE WORMS and the TTJRBELLARI^E. 



* Quoy et Gaimarcl, Voy. de 1' Astrolabe. 



