TWO I'ARASITIC TOEMS OF THE FAMILY TETRAKIIYNCUIDiE. 333 



through the neck into the head, as vrill be subsequently detailed. 

 The visceral space averages ^^ inch in thickness, and occupies the 

 centre of the segment (fig. 3,/; fig. 4,/). It contains a granular 

 homogeneous albumenoid material with a few interspersed inor- 

 ganic particles ; and although not presenting, in the segments ot 

 any of the three parasites from which these details are taken, any 

 trace of viscera, yet, following by analogy the developmental pro- 

 cess in the allied, family of Tapeworms, it is within this, and 

 from this, that the generative viscera are produced. At each 

 lateral end of the visceral space, at a distance from the sides 

 of the segment of ^^ inch, is seated the cut lumen of the longi- 

 tudinal water-vascular canals (fig. 3, g). These canals, yto ^"^^ 

 in diameter, are mere channels in the substance lined by a deli- 

 cate fibrous-tissue layer ; the longitudinal ones pass from one seg- 

 ment to the other, and, as in the Tcsnia mediocanellata, are met by 

 a transverse branch special to the segment, seated in the lower 

 portion of the visceral space of each zooid — the combination ot 

 the two series of tubes giving to the system of vessels of the 

 colony the aspect of a ladder. The transverse branches are oval 

 in outline, and smaller than the longitudinal branches ; they ne- 

 cessarily approximate closely in the front segnients of the colony 

 (fig. %g), and diverge more and more from each other in the pro- 

 gressive development towards maturity. 



The only other feature of the segments to be noted is the con- 

 tracted bulbous condition of the lowermost one above referred to. 

 On section under the microscope this gave the structure and ap- 

 pearance of a collapsed vesicle thickly studded with " calcareous 

 corpuscles ; " and the inference drawn from it and the lower end 

 of the colony is, that these parasites were in the early progress of 

 growth towards maturity from the larvae recently introduced into 

 the stomach of the shark, and that the nodular free end was the 

 remnant of the original vesicle not yet, in two of the parasites, 

 thrown off, while in the third, as above mentioned, this had en- 

 sued. I may here state, too, another feature which appears to 

 bear out this deduction. In one of the animals one of the diver- 

 gent petaloid appendages of the head arranged in a circle at the 

 base of the proboscides was present, w^hile in the other two ani- 

 mals there w as no trace of them nor of the three others in the 

 same parasite. These four appendages appear to be common among 

 the mature Tetrarhynchs, and more especially among the larvaB ; 

 and the existence of one under the circumstances mentioned would 



