The life history of Diplodiscus temporatus Stafford. 643 



the lumen extends into the arms forming- two flask-shaped cavities. 

 The backward growth takes place from these extremities so that 

 the caeca are developed in their normal positions. 



The lumen of the pharynx and oesophagus is established in 

 the same manner as was that of the gut. The oesophagus, which 

 w^as at first very short, keeps pace with the lengthening- of the 

 body. The development of the muscular, posterior, enlargement of 

 the oesophagus takes place after the young cercaria has reached 

 its characteristic form and the parts of the digestive tract have 

 acquired their proportionate lengths. 



The anterior sucking disc is formed from some of the cells 

 which were cut off from the remainder of the body when the pri- 

 mordium of the pharynx and oesophagus was marked out. Schwarze 

 describes the formation of this sucking disc from cells which become 

 cut off from the surrounding ones by a thin membrane. His figures 

 (fig. 5, 10, 12 and 15) of the anterior sucking disc show it in rather 

 late stages of development only; but on comparing fig. 4 and 5 one 

 is lead to the conclusion that it must have originated from the 

 primordium wiiich in fig. 4 is labeled "fZ" (= primordium of the gut). 



The cells concerned in the development of the anterior sucker 

 undergo changes very different from those in the posterior part of 

 the same primordium. In the former cells there appear a large 

 number of fine fibres, wiiich gradually come to fill the greater part 

 of the cell, which is thus transformed into a muscle cell. From 

 their original scattered arrangement they come to lie perpendicu- 

 larly to the surface of the disc, which in its later development 

 becomes strongly concave. In the dorsal part of the sucker there 

 is developed the dart (-'Stachel"). This lies in a thin structureless 

 sheath between the muscle cells. It is shaped like a short arrow 

 with a comparatively broad head. 



The dart is made up of an outer hyaline, highly refractive, 

 layer and an inner dark, homogeneous, layer. According to Schwarze, 

 with whose observations my own agree, this organ is developed by 

 intracellular cytoplasmic differentiation. 



The development of the water vascular s 3^ stem. 



As regards the development of the water vascular system in 

 cercariae two views have been set forth w^hich are decidedly opposed 

 to one another. Loess (1892), in describing the development of 

 the cercariae of Ampkistomum siihckwatnm, says that the canals arise 



