376 DE. P. MANSON ON THE METAMOEPHOSIS OF 



tegumentary set of cells and those forming the alimentary canal there is a distinct 

 interval, a sort of peritoneal cavity, in which at a later stage the alimentary tube moves 

 freely, not being attached at any point save at the anus and mouth (fig. 25). 



At the conclusion of this, the third stage of the metamorphosis, the Filaria measures 

 from xno" to ^ -" in length, by gro" *° sou" i n breadth, or thereabouts. There is considerable 

 diversity both in size and shape. The mouth is wide open ; the tail is large and sickle- 

 shaped, and the cells of the body usually dip into it. The alimentary line runs from 

 mouth to anus ; and the cellular nature of the entire animal, with the exception of the 

 integument, is easily demonstrated. Motion is entirely suspended. 



Growth hitherto has been very slow, but now, when the fourth stage commences, it 

 becomes rapid, the animal quickly attaining a length of from a seventieth to a fiftieth of 

 an inch. The walls of the alimentary canal can be distinctly traced as the walls of a tube 

 from the open mouth to near the anus, but not quite into structural continuity with the 

 latter. The cellular structure of the mass of the body is often beautifully distinct (figs. 

 23, 24, 25, 26). The body retreats from the tail, which becomes a mere empty integu- 

 mental appendage, so transparent that it can with difficulty, in many cases, be made out 

 (fig. 34, 35). The addition of water to the preparation causes the little animal often 

 to rupture after a few minutes at one or two points, a cloud of cells and granules escajfing. 



In not a few instances a vacuole, similar to that which preceded the formation of the 

 anus, is seen at this stage at a point some little distance behind the mouth. This may 

 be the result of endosmosis, but possibly it is the forerunner of vulva and organs of 

 generation. In one instance (fig. 23) I detected a line among the cells which, from its 

 position, forcibly suggested that it was the rudiment of organs of generation ; but as 

 this was a solitary observation, I attach no great weight to it. The vacuole, in the 

 position I mention, is quite a common occurrence. 



When the body has attained its maximum thickness the fifth stage commences. The 

 cells, especially in the anterior part, gradually lose their distinctness, and the mouth 

 inclines to purse up, while the animal, as a whole, elongates and attenuates. In some 

 the lengthening and thinning begin at the head and extend backwards, so that if we 

 find such a Filaria in the middle of this extension-process, it has a narrow anterior half 

 rather abruptly swelling out into a thick posterior half with the dimensions of the 

 previous stage (fig. 27). Such a specimen resembles in shape a hock-bottle. A very 

 few elongate anteriorly and posteriorly simultaneously ; and we may find in one under- 

 going this process a small segment of the middle of the body still thick and unextended 

 (fig. 28). More generally, however, the extension-process occurs simultaneously 

 throughout the body, the fore part being always rather in advance of the rest (figs. 29, 30). 



Sometimes at this stage, just before the mouth closes up, the alimentary tube is seen 

 very distinctly. It moves freely in the body-cavity, accommodating itself to the gentle 

 movements which the anterior half of the Filaria now exhibits. The pharynx in such a 

 specimen is distinctly indicated by two short parallel or curved dark liues, terminating at 

 one end in the mouth, at the other leading into the oesophagus (fig. 44). At its other 

 extremity the oesophagus ends in a thick bulbous valvular-like arrangement opening 

 into the intestine, which in its turn may be traced as a distinct tube almost to the anus 



