192 Robert Thompson Young, 



c) Cuticiila and Hooks. 



The formation of the cuticula occurs simultaneous!}' over the 

 entire bladder before the invagination of the head cavity commences. 

 In accordance with the development of the larva, the cuticula is 

 soon found in a more advanced stage in the head and neck, while 

 in the bladder it occurs in an embr3^onic state. In a larva in which 

 the invagination of the scolex cavity has just commenced, the 

 cuticula is already well ditferentiated as a layer of nearly uniform 

 thickness and structure over the entire surface of the worm, except 

 in the head cavity where it is thicker and of a much looser structure. 

 Beneath tlie cuticula lie the transverse and longitudinal fibres of 

 the sub-cuticular muscles, as well as parenchyma fibres passing 

 outward perpendicular to the surface. The latter are the rudiments 

 of the outer processes of the futui-e sub-cuticular cells and the 

 parenchyma fibres lying between them. Careful observation with 

 high magnification shows that these fibres extend into and form the 

 groundwork of the cuticula, in which they divide into fibrillae which 

 run in diverse directions, as described by Schneider (1902). Between 

 these fibrillae is a homogeneous, translucent cement substance laid 

 down by them. From the visible continuity evident at this early 

 stage between the processes of the future sub-cuticular cells and 

 the groundwork of the cuticula, a ready explanation is found for 

 this connection as it exists in the adult. Soon after the first 

 appearance of the cuticula, it begins to scale ofl:' on the surface and 

 is continually renewed at its base. There are no marked periods of 

 ecdysis, but rather a continual shedding of the cuticula, which may 

 be seen partly filling the head cavity at about the time the hooks 

 begin to appear.^) 



The pieces of shed cuticula present a granular and vacuolated 

 appearance and this appearance is often visible in the cuticula when 

 still attached to the larva. The second stage in the development 

 of the cuticula is marked by the ditferentiation of an outer and an 

 inner laj^er. The relative thickness of these two layers varies at 

 first, but in later stages the outer becomes much thinner and may 

 even be entirelj?^ lost. The inner layer alone contains the cement 

 substance of the cuticula. The fibrillae which make up the ground 

 work of the inner layer are continuous with the fibrillae or "hairs" 



1) An active ecdj^sis is not seen anywhere except in the head cavity. 



