CESTODES EXAMINED 381 



In the species of Tetrabothrius examined during the present work, the testes are situated dorsally, 

 except in T. affinis, where they are distributed dorso-ventrally. This arrangement, however, may be 

 altered and affected by the growth of the uterus, particularly when filled with eggs. 



The testes seem also to decrease rapidly in number in the gravid segments, being either partially 

 spent or else crowded nearer to the surface of the medullary parenchyma by the uterus. Some differences 

 in the size of the testes in mature segments (measured in transverse sections of the body) are notice- 

 able, and this may be regarded as a valuable feature in specific differentiation (Table 1, p. 387). 



The cirrus-sac, although similar in structure, is also subject to much contraction and its size and 

 shape depend greatly on this factor. In some cases, as in Trigonocotyle globicephalae, and 7V. prudhoei 

 sp.n., some muscular modification occurs in the area of the genital atrium, forming a kind of sucker. 

 This, however, is also met with in other species of Tetrabothriids not found in whales. 



In cetacean Tetrabothriids, the ovary and vitelline glands are situated in the ventral half of the 

 segment. In some of the species examined, the ovary contains fairly large egg-cells. The full develop- 

 ment of the ovary is reached in the portion of the strobila occupied by the mature proglottids. From 

 this point onwards towards the posterior end, the ovary gradually undergoes a process of deterioration, 

 caused by the overgrowth of the uterus. 



In fully gravid segments the uterus is so strongly developed that it practically replaces the rest of 

 the genital system. It is a centrally situated, sac-shaped organ extending laterally and gradually 

 occupying the whole of the medullary parenchyma. 



There is no proper uterine pore in cetacean Tetrabothriids. Instead, the uterus opens through the 

 body-wall, when the eggs are ready to be discharged from the segment. The rupture through which 

 the eggs are passed appears on the dorsal surface of the body, at one definite point, and the path in the 

 cortical parenchyma, along which the distending uterus is pushed, is surrounded by a distinct mass of 

 eosinophil tissue. In Priapocephalus grandis, however, many such ruptures on the dorsal surface have 

 been observed. No uterine openings or ducts have been found in the two species of Trigonocotyle 

 from the present collection. 



It seems that a kind of ' cement M protects the newly formed uterine opening from the effects of the 

 gastric juices in the host's intestine and which closes the opening after the eggs have been discharged. 



It is well known that the eggs of cestodes lose much of their characteristic structure after fixation and 

 extraction from the uterus. Nevertheless, specific differences seem to occur between the eggs of particular 

 species found in the Discovery Collection. Their size appears to be related to the thickness of the outer 

 and inner membranes in the various species, and in some cases to the number of membranes as well. 



The embryos also seem to vary in size with species, and, with the exception of Tetrabothrius ruudi 

 (Fig. 15), the three pairs of hooks with which they are all provided exhibit specific differences (Table 1). 



The vagina runs ventrally to the cirrus-sac in all examined species of Tetrabothrius. 



The most striking differences among particular species seem to occur in the structure and arrange- 

 ment of longitudinal muscles, as seen in transverse section. 



In the present material, the longitudinal muscles of Tetrabothriids undergo morphological changes 

 within the individual strobila itself. These changes are associated with the development of the genital 

 organs and it appears that in the immature portion of the strobila, where the genital organs are either 

 non-existent or rudimentary, the longitudinal muscles are differentiated into bundles, but in the mature 

 portion of the strobila, these bundles have taken on a very definite arrangement. As the mature proglot- 

 tids ripen, and the uterus becomes increasingly distended, the muscle bundles tend to atrophy, although 

 their characteristic arrangement can still be distinguished. This series of changes is shown in sections of 

 Trigonocotyle globicephalae (Figs. 24, 25) and Priapocephalus grandis (Figs. 34-36). 

 1 This 'cement' is probably produced by gland-cells in the modified tissue. 



