120 INVERTEBRATE ZOOLOGY 



bears a dilated portion for the storage of sperm cells called the 

 receptaculum seminis or seminal receptacle. As the duct con- 

 tinues beyond this receptaculum it receives two side branches, 

 the oviduct which connects it with the ovary and the vitelline duct 

 which communicates with the vitellaria or vitelline glands. The 

 oviduct frequently bears a modification called the oocapt for 

 forcing the eggs along their course. When eggs and spermatozoa 

 meet, fertilization occurs. The fertilized eggs and the substance 

 from the vitelline glands pass into a structure similar to the 

 ootype of the trematodes in the region of the shell gland. Here 

 shells are formed around the eggs which are then passed on into 

 the uterus. In older proglottids, the uterus frequently develops 

 egg-filled pouches which occupy practically the entire volume of 

 the proglottid. Eggs are rarely discharged from the uterus, for 

 more frequently the entire egg-filled proglottid is liberated and 

 carried out of the body of the host. 



Other Organs. — The nervous and excretory systems extend the 

 length of the strobila and parts in individual proglottids do not 

 represent complete units. Ganglia located in the scolex send 

 nerve trunks backward through the proglottids, usually as two 

 lateral branches fairly close to the lateral margins of the body. 

 Specialized sensory organs are wholly wanting. Parallel to the 

 longitudinal nerve trunks run the main canals of the excretory 

 system. These in turn receive smaller canals the branches of 

 which ramify through the parenchyma and terminate in flame 

 cells. 



Reproduction. — Several different methods of development 

 occur in the Cestoda. Typically, the egg undergoes a cleavage 

 within the shell to form an embryo, bearing six minute hooks, 

 called an onchosphere. The onchosphere may in some instances 

 be provided with cilia for locomotion (as in Diphyllobothrium, 

 the fish tapeworm of man), but more frequently it is borne within 

 embryonic membranes as an immotile body. Upon introduction 

 into a suitable host, the onchosphere develops into a larval form 

 differing with the various groups of the cestodes. Among the 

 lower cestodes, which live chiefly in fishes, this larva is a small 

 solid body called a plerocercoid (Fig. 68 A). A larval stage 

 called the cysticercus or bladderworm stage (C) is found among 

 the higher cestodes. The name cysticercoid is applied to larval 

 forms {B) which seem to be intermediate between the plerocercoid 

 and the cysticercus. 



