LEIDY'S DESCRIPTIONS OF DISTOMA. 305 



Second stage. (Fig. 2.) — Body oval or subrotund, smooth, beneath the epidermoid 

 covering fasciated. Oral acetabulum placed beneath the anterior apex of the body, 

 1-I60th in., with a double oral orilice : external aperture triangular ; interior 

 round or elliptical. Oesophageal bulb oblong, l-296th in. long by l--222d in. broad. 

 Oesophagus short. Intestinal rami tortuous. Ventral acetabulum orbicular, l-174th 

 in. in diameter, vi'ith an orifice opening into an excavation in its interior ; orifice with 

 a thickened margin, transversely elliptical, or round when most dilated. A third 

 orifice, generative, situated a little more than one third the distance between the 

 ventral acetabulum and the posterior extremity of the body. Vascular system 

 consisting of a large double tortuous trunk on each side of the body commencing in a 

 bulbous dilatation, about the l-666th in. in diameter, near the middle line of the body 

 posteriorly. Furnished with a posterior contractile sac communicating with the 

 dilatations of the vascular canals, and with the exterior. Length of the sac l-195th 

 in., breadth, when fully dilated, l-470th in. 



Whole length l-30th in., capable of elongating to l-20th ; breadth l-42d in. 



Third stage. (Fig. 8.) — A.nimal more active, usually elongated. Posterior contractile 

 sac remaining permanently contracted, and cut off from the vascular canals. The 

 latter consisting of four principal vessels on each side of the body, forming a very 

 tortuous and intricate plexus. The principal or largest of the four vessels, furnished 

 from its commencement posteriorly to where it turns upon itself anteriorly with vibrillee. 



Remarks. — During the earlier two stages of the existence of Distoma vagans in 

 Helix alternata and H. albolahris, it is found within the pericardial cavity bathed in 

 the liquor pericardii, and never in cysts, or in other organs of the body. The 

 Distoma in the second of the earlier stages is the most frequently Observable, and 

 indeed it is a rare exception to find a specimen of H. alternata without it. In one 

 instance I found as many as fifty-two individuals of the second stage together with 

 four of the first stage in the latter species of Helix, which is an enormous number, 

 when we consider the size of the pericardial cavity, for it measures only two lines in 

 length by one in breadth, so that the cavity was distended by these parasites, and 

 probably they impeded to some extent the action of the heart of the animal. The 

 usual numbers found are from one to a dozen. In the first stage the Distoma is more 

 active than in the second : in the latter being sluggish and only elongating or 

 shortening upon irritation or pressure. Usually they lie loosely bathed in the liquor 

 pericardii, but occasionally they are found hanging by means of the oral acetabulum 

 to epithelial shreds attached to the surface of the heart. Just before entering into the 

 third stage, they are almost always found in the last mentioned condition. The third 

 stage is not of such frequent occurrence, or more properly it is not so readily 



