DEVELOPMENT OF THE HEAD IN THE CYSTICEECOIDS. 



653 



times finely strealted cuticle, on which in some cases the embryonic 

 hooks still lie, and under which there is a finely cellular parenchyma, 

 often penetrated by numerous fat globules. Since it is contractile, it 

 probably contains muscle-fibres. Calcareous corpuscles are, it seems, 

 found only in the appended head-process. Similarly, the ramifications 

 of the vessels appear to be exclusively confined to the latter, although 

 the longitudinal stems are seen to traverse the bladder, and to open 

 by a foramen at the lower end. 



In contrast to these common forms of Cysticercoid, there are 

 others in which the sac-like outer body in appearance and structure 

 resembles a simple continuation of the head-bearing anterior body, to 

 which it stands almost in the same relation as the posterior portion of 

 the JEchinococcns-he3i.d to the anterior end, which bears the attaching 

 apparatus. We have already (p. 364) noticed this formation, especially 

 in the Cysticercoids of Tcenia cucumerina, and have attempted to 

 show that this outer body was, in spite of its divergent structure, still 

 nothing but the caudal bladder, i.e., the first developmental product 

 of the six-hooked embryo. The morphological individuality (p. 386) 

 has indeed been lost ; head and caudal bladder appear as a unit, whose 

 development presents rather the appearance of a metamorphosis than 

 of an alternation of generations. 



Fie. 338.— Cysticercoid Fig. 339.— Young form of Tcenia torulosa (?) 



of Tcenia cucumerina,. in Cyclops serrulatus, after Gruber. ( x 25.) 



(x 60.) 



This is perhaps yet more distinct in the Cysticercoids from the 

 Cyclops discovered by Gruber, where the head is always protruded, 

 instead of being retracted into the posterior body, the characteristic 

 form of the bladder-worm being thus disguised. 



■ As in the development of the caudal bladder, so in the position of 

 the head there are manifold differences, and that even when we leave 

 those with free head out of account, and confine ourselves to those 

 species in which the head is retracted into the body. 

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