337] PSEUDOPHYLLIDEA PROM FISHES— COOPER 49 



Thus there is a tertiary subdivision of the secondary segments, which must, 

 however, be considered as by no means as regular as the secondary subdivision 

 of the original primary segments. These facts explain the aberrant nature of 

 the strobila in this region, noted formerly by the writer, and the presence in 

 material of chains showing anteriorly very yoimg segments similar to those 

 formed in the oldest attached secondary strobilas but posteriorly much older 

 segments with well-developed auricles and farther back the typical mature 

 proglottides of the ordinary strobila. Consequently it is probable that there 

 is not a definite number of segments formed, but that further, irregular and 

 evidently indefinite subdivision, resulting in the formation of an inconstant 

 number, takes place chiefly in the middle portion of the anterior segmented 

 region of what now must obviously be called the secondary strobila. 



In the hght of this method of segmentation certain facts in connection with 

 the nervous and excretory systems that were previously considered to be very 

 unique, to say the least, will now be expUcable. In primary strobilas, even 

 those that are youngest, the excretory system consists, as in the adult, of a 

 larger median vessel and two lateral vessels which run backward and unite 

 in the posterior end to form a plexus from which very many small vessels 

 pass to the exterior by prominent foramina secundaria piercing the cuticula, 

 much as described by Fraipont (1881:11, Fig. 7, PL II) for Bothriocephdus 

 scorpii. In the youngest larva I have been able to find only the median 

 vessel, which becomes greatly reduced in diameter about twice the length 

 of the bulbs from the anterior end is present in the scolex. It forms a simple 

 plexus among the bulbs anteriorly. In primary strobilas, however, in which 

 segmentation has gotten well under way, all three vessels are quite prominent. 

 They pass close to each other as well as to the chief nerve strands, when they 

 traverse the constrictions between the developing secondary strobilas, where the 

 median vessel is somewhat enlarged. As they near the anterior end of the 

 worm they give off numerous branches of their own calibre, and when they 

 meet the large ganglionic mass described below, diverge as four vessels (two 

 on each side) and continue lateral to the bulbs to the tip of the scolex. Here 

 after forming an open plexus among the anterior ends of the former, they unite 

 in a single median frontal loop. As the constrictions between the secondary 

 strobilas deepen all three vessels likewise become gradually constricted until 

 eventually they are cut off, and the adult conditions are subsequently developed 

 by the simple process of the turning in of both severed ends. And since in 

 these yoimger forms the median vessel is considerably enlarged at the region 

 of constriction, it remains thus in the hinder end of tlie adult strobila as weU 

 as in the first segment — as described and figured elsewhere by the writer 

 (1914a :93, Figs. 12, 37) — ^while in the latter it is joined by the lateral vessels 

 to form the characteristic terminal vesicle. 



The nervous system of the primary strobila consists of two chief strands 

 passing thniout the segments, a quite irregular commissure connecting them 

 anteriorly, and a very large ganglionic mass situated some distance posterior 

 to the proboscis bulbs. The chief nerve strands, which are quite indistinct 



