PARASITIC FLATWORMS 451 



126(125) Four suckers on the scolex of the larva. . Cysticercus . . 127 



The head of the larva is inverted so that these suckers are in the center of the mass and may 

 easily be overlooked. In the narrower sense the term cysticercus is applied to the large thin- 

 walled bladder-worm having a cavity of considerable size filled with watery fluid in which the 

 scolex grows from a polar papilla that subsequently hollows out giving in reverse the scolex of the 

 adult. This larva belongs to the terrestrial fauna and occurs only accidentally in aquatic forms 

 like the muskrat which have become a part of the aquatic fauna secondarily. 



127 (128) Entire larva solid parenchyma tissue. Scolex invaginated with 

 apex at bottom of infolding Plerocercoid. 



Caudal region not differentiated at all or only very poorly indi- 

 cated. In general structure the Bothriocephalid larvae with two 

 sucking grooves are like the true plerocercoids with 4 acetabula. 



Larvae prominently of Proteocephalidae, also of Cyclophyllidea 

 from reptiles. One special form known as Gryporhynchus has been 

 identified as the larva of Dilepis or some allied genus. 



FIG. 763. Proteocephalus plerocercoid; a, from the body cavity; b, from 

 cyst, suckers drawn within body. Note large end organ, shaded; c, from 

 intestine; optical sections, X 10. (After La Rue.) 



128 (127) Spaces in larva between folds of tissue. Scolex in natural position, 

 surrounded by cyst * . , . . . . Cysticercoid. 



Usually with caudal appendage in a posterior hollow of the cyst, and on the tail the embry- 

 onic booklets of the onchosphere. In form and texture the cyst varies greatly. Calcareous 

 bodies abundant, mostly on the invaginated layer between the cyst and the scolex which corre- 

 sponds to the neck when the larval head is evaginated. 



Very frequent among Cyclophyllidea. Relationship between the cysticercoid and the adult 

 may be inferred from careful examination of the scolex and its armature. 



No records exist of the 

 presence in North Ameri- 

 ca of these forms. For 

 convenience in recogniz- 

 ing them a figure is in- 

 cluded of an abundant 

 European form likely to 

 occur here also in similar 

 aquatic hosts. While 

 these cysticercoids are 

 most frequently recorded 

 from Copepoda, Ostra- 

 coda, and other small 

 aquatic Crustacea, they 

 occur also in Lumbricu- 

 lus and other small an- 

 nelids, and more rarely in 

 c small snails and slugs. 



FIG. 764. Drepanidotaenia 

 fasciata. a, body of Cyclops 

 agilis, containing larval 

 tapeworm (cysticercoid); b, 

 same larva with enormously 

 long tail, isolated from the 

 crustacean; c, cysticercoid 

 with extended head, mag- 

 nified. (After Mrazek.) 



