io COMPARATIVE ANATOMY 



The eyespot consists of two crescentic cells containing 

 pigment, placed with their convex sides turned towards each 

 other, and their anterior horns in contact. The concavity of 

 each cell is filled with a refractive material which serves as a 

 lens. The double eyespot is situated on what may be called 

 the dorsal side of the body, just behind the anterior ring of 

 ciliated cells. The nephridia are in the form of a pair of 

 flame-cells placed one on either side of the middle of the' 

 body. 



The centre of the body is occupied by a mass of large 

 rounded cells, which, because of their subsequent history, are 

 called germinal cells. There is no gut and no endoderm, 

 unless a rounded granular mass lying just behind the anterior 

 papilla is to be regarded as representative of such. 



The miracidium larva is exceedingly active, and swims 

 restlessly about, testing all objects which it comes in contact 

 with, and leaving them if they are not suitable to its purposes. 

 But if it comes in contact with a certain fresh-water snail, 

 Limncea truncatula, it at once begins to bore its way into its 

 tissues. The larva presses its anterior end against the tissues 

 of the snail, protrudes its papilla, and forcing it like a gimlet 

 into the tissues, it spins round and round by means of its 

 cilia until it works its way into the body of its new host. It 

 is a remarkable fact that the larva of D. hepaticum selects the 

 particular species of snail, Limncza truncatula^ in preference 

 to any other ; and if no members of this species are to be 

 found, its activity gradually diminishes, and eventually it dies. 

 The limit of the free existence of a miracidium is about eight 

 hours. When the larva has forced itself into the snail's body, 

 it undergoes a metamorphosis, provided that it has made 

 its way into a suitable region. The firm, muscular " foot " 

 of the snail is inimical to further development, and although 

 larvae may make their way into it, they do not long survive. 

 But in softer regions, such as the pulmonary chamber, they 

 are changed into a sac-like form known as the sporocyst. 

 The outer coat of ciliated cells is thrown off, the eyes separate 

 from one another and lose their crescentic form, and the 

 organism becomes a mere sac. During these changes, which 

 in warm summer weather take about a fortnight, the sporocyst 

 grows considerably in size ; at first it was only about '07 mm. 

 in length, but eventually it attains a length of -5 7 mm. 



