384 DISCOVERY REPORTS 



The uterus is a well-developed sac. There is no special uterine pore, but dorsally in the central 

 part of the segment, the parenchyma undergoes some modification. Transverse sections of this part of 

 the segment show differentiated tissues which appear to form a cone. Gradually the distending uterus, 

 filled with eggs, breaks through this part of the segment and the eggs are discharged through the rup- 

 ture in the body-wall. The diameter of the egg is about (66-106x33-83)^ with the embryonic 

 membrane measuring 51 x66it. The embryo measures about 52x2611 and is provided with three 

 pairs of hooks, of which the lateral pairs measure 17 /a in length, and the central pair 20 11. 



The excretory system, composed of two main canals with transverse vessels, is very well developed. 

 The ventral canal is relatively wider in diameter than the dorsal one, but the latter possesses thick 

 walls. 



Tetrabothrius wilsoni (Leiper & Atkinson, 1914). (Figs. 7-10.) 



Oriana wilsoni Leiper & Atkinson, 1914. 

 Tetrabothrius wilsoni Baylis, 1926. 

 Tetrabothrius affinis Baer, 1954. 



Hosts: Sei Whale (Balaenoptera borealis); Blue whale (B. musculus). 

 Localities : Durban ; South Africa ; South Georgia. 



Of the thirty-four specimens examined several appear to be gravid, a condition not hitherto 

 reported. The length of the worms varies from 6 to 14 cm. and the width from 2 to 3 mm. The body 

 is dorso-ventrally flattened. The scolex bearing four suckers with weakly developed 'epaulettes' is 

 about 3 mm. broad. 



The longitudinal muscles are arranged in two very indistinct rings, as shown in Fig. 9. Individual 

 bundles are not well defined and are composed of rather thick muscle fibres. Their arrangement 

 appears diffuse and differs considerably from that found in Tetrabothrius affinis. The circular muscles, 

 though present, are poorly developed. 



The testes are situated in the dorsal part of the segment, being 99 x 120 11 in transverse section. 

 They seem to be arranged in an irregular layer. The vas deferens is not very strongly developed, while 

 the cirrus-sac is pear-shaped or more or less spherical in transverse sections of the strobila. 



The ovary is extremely well developed, lobate, extending transversely in the ventral part of the 

 segment, and occupying a considerable area of the proglottid. 



The egg-cells are fairly large, being about 1514 in diameter. 



The vitelline gland, situated ventrally beneath the ovary, is well developed. 



The uterus, in fully gravid segments, occupies the whole of the medullary parenchyma of the seg- 

 ment, considerably displacing the internal organs. The walls of the segments are much thinner than in 

 T. affinis. 



The eggs are discharged through a small opening in the dorsal face of the segment. They measure 

 about 90 x 60 ix. The embryonic membrane is about 45 x 30/x and the embryo 36 x 21 /x in diameter. 

 It seems that the latter is attached by its membrane to the external egg-capsule, as shown in Fig. 10. 

 The external and internal pairs of embryonic hooks are of similar length, i.e. about 12 /a. 



The excretory system appears to be weakly developed and both the ventral and the dorsal trunks are 

 of similar diameter. In the younger segments, as well as in the gravid, the ventral trunk of the excretory 

 system seems to have a greater diameter. This, however, may be caused by the fixation and con- 

 traction of the tissues. 



This form, described by Leiper & Atkinson in 1914, by Baylis in 1926 and recently by Baer (1954), 

 represents a valid species, distinguished by differences in the arrangement and structure of the longi- 



