20 ILLINOIS BIOLOGICAL MONOGRAPHS [20 



todes. Likewise a study should be made of certain species of Cyclophyl- 

 lidea which are known to possess a rudimentary or a vestigial rostellum. 

 No definite solution of this problem can be reached without such a com- 

 parative study. Data as to the presence or absence of the fifth sucker 

 in species of the group, whether functional or vestigial, is to be found 

 in the tables of the principal characters of the cestodes of this family. 



The nervous system is made up of a nerve ring which occurs in the 

 neighborhood of the suckers and one pair of main lateral nerve trunks 

 in the strobila. As described by the writer (1909) for Ophiotaenia fila- 

 roides the nerve ring is somewhat octagonal in shape and at its corners 

 nerve processes extend out to the suckers. From the ring main trunks 

 extend back into the strobila. In the strobila a single pair of main 

 lateral trunks may be seen just within the lateral fields of the muscle 

 sheath. No accessory nerve trunks have been made out by the writer. 

 In Ophiotaenia filaroides the nerve trunk passes dorsal to the cirrus and 

 the vagina. 



In all species the neck is a more or less poorly defined unsegmented 

 region between the head and the segmented part of the strobila. This 

 region may be several millimeters long in certain species while in others 

 it is said to be entirely lacking or may be no more than 0.3 to 0.5 mm. 

 long. It is usually narrower than the scolex tho in exceptional cases in 

 which the neck muscles may be unduly contracted the neck is broader 

 than the scolex. Much confusion has arisen over the statements of the 

 older investigators who described the neck of certain species as being long 

 or very long without, however, giving measurements. These adjectives 

 are relative terms. If the cestode is small, 0.5 to 1 mm. broad and 20 to 

 100 mm. in length, a neck which is 1 to 5 mm. long seems long or very 

 long. A neck of that length on a cestode as large as Crepidobothrium 

 gerrardii which is several millimeters broad and many centimeters long 

 would be called short. In this species an unsegmented region 0.5-1 mm. 

 long is ignored by some authors and the specimen is reported as having 

 no neck. It is essential that actual measurements of the unsegmented 

 region be made. For this purpose stained specimens are necessary be- 

 cause unstained specimens will not show segmentation plainly and also 

 because external wrinkles or folds may simulate segmentation. In re- 

 cording measurements of the neck the writer takes the length from the 

 point of narrowing behind the suckers to the first evident traces of in- 

 ternal segmentation. In a few species another system has been found 

 necessary but in these cases the method of measuring has been stated. 

 The breadth given is the narrowest place in the unsegmented region. 



The segmented part of Proteocephalids varies greatly in length, 

 breadth, and thickness. In some species the strobila is small, being not 



