32 PROCEEDINGS OF THE NATIONAL. MUSEUM. vol.64. 



The inner end of the horizontal bar projects slightly beyond the 

 inner prong and meets its fellow on the median line of the bothrium. 

 Under favorable conditions a small papilla may be seen between the 

 anterior ends of the bothria (fig. 37). 



Strobile. — The neck, and sometimes the proglottides, are armed 

 with spines. Seen in front view these spines are triangular. In 

 some cases they are very abundant. They are, however, evanescent, 

 and specimens may be encountered from which they have disap- 

 peared entirely. The neck is linear and usually rather straight, in- 

 creasing in breadth gradually and uniformly. The first indication 

 of proglottides appears from 2 to 3 mm. back of the scolex. The 

 first proglottides are broader than long, or squarish, but soon become 

 longer than broad, and adult proglottides are much longer than 

 broad, and somewhat rectangular in outline. 



Reproductive organs. — The genital apertures are irregularly alter- 

 nate, and are situated at about the middle of the lateral margins. 

 When an adult proglottis, in which ova have not yet made their 

 appearance, is observed in dorso-ventral view the ovary is seen to 

 occupy about the posterior fourth, the vitellaria forming a dense bor- 

 der along each lateral margin. The vagina lies close to the anterior 

 border of the cirrus-pouch, runs at nearly right angles to the long 

 axis of the proglottis to the median line, thence in a sinuous course 

 to the ovary. The rudiment of the uterus lies along the median line 

 and extends to near the anterior end of the proglottis. The cirrus- 

 pouch is somewhat pyrif orm, and is nearly at right angles to the mar- 

 gin ; its length is approximately equal to one-third the breadth of the 

 proglottis. A little of the vas deferens is enclosed in the inner end 

 of the cirrus-pouch, thence its rather voluminous coils extend forward 

 along the median line. The most conspicuous organs are the testes, 

 which fill the interior of the proglottis in front of the ovary within 

 the layer of vitellaria. In horizontal sections the testes are circular 

 in outline; in transverse sections they are elliptical, and lie in a 

 single row 



Until the summer of 1922 the ova of this species were not certainly 

 identified. Then I had the opportunity of examining a large num- 

 ber of sharks, and succeeded in demonstrating the presence of rod- 

 shaped ova (fig. 54) in gravid proglottides of P. lasium. These were 

 at first taken to be linear clusters of eggs, but they seem to be eggs 

 which are undergoing segmentation. Of those observed the vast 

 majority consisted of 4 cells, a few were seen with 3, many with 5, 

 and one with 6 cells. In all cases the cells were in linear arrange- 

 ment. When proglottides, plump with ova, are placed in sea water 

 they soon burst and a chalky white mass of ova is discharged. 



Three species of cestodes, of practically similar dimensions, and 

 of superficially like appearance, were found often associated to- 



