116 A. r. THOMAS. 



all, there is visible beneath the cuticle a finely granular layer 

 in which the muscle-fibres appear to be embedded. 



It appears probable that some of the most superficial cells of 

 the body, or at least portions of them, are converted into 

 muscle-fibres, whilst others undergo more or less degeneration. 

 These muscle-fibres are more feebly developed in the sporo- 

 cysts than in the redise which form the next asexual genera- 

 tion, and in accordance with this feeble development the sporo- 

 cysts are exceedingly inert, and rarely show any movements. 

 In the redia active movements are necessary in order that the 

 digestive tract, which is present, may be filled with food, and 

 the muscular system accordingly reaches a greater development. 

 In the sporocyst the digestive tract is altogether rudimentary, 

 and as no exertion is required to procure the nourishment, the 

 degeneration of the muscle-fibres has to some extent followed 

 that of the enteron. In those sporocysts, however, in which 

 the power of performing active movements is useful for some 

 other reason, the muscles may retain a high degree of develop- 

 ment; as is, for instance, the case in the sporocysts of Cer- 

 caria limacis, which, as soon as their included cercariee are 

 sufficiently matured for transference to the ultimate host, bore 

 their way out, through the thick integument of the slugs (Arion 

 ater and Limax cinereus), which serve as intermediate hosts, 

 and are then left behind in the mucous track ofthe slug. 



Immediately following the layer of muscle-fibres is an epi- 

 thelium, which lines the cavity of the sporocyst, and forms the 

 greater part of the thickness of the body wall. It is composed 

 of cells of very various sizes, round or polygonal in form, and 

 containing large nuclei (fig. 11). The layer in most places is 

 only one cell deep, though adjacent cells may overlap one 

 another ; but in places, and especially in the less mature sporo- 

 cysts, it is two or three cells deep. The excretory system is 

 lodged in the body wall of the sporocyst; on each side may be 

 distinguished an irregular group of about half-a-dozen ciliated 

 infundibula. They have the same structure as the two described 

 above as being present in the embryo, and are always found in 

 the middle third of the length of the body. No clearly defined 



