The postlarva has been found in the following host species:

The most common site of the parasite is in the body cavity of the host. In a specimen of Lepidopus caudatus (Euphrasen), nine postlarvae were found in the body cavity, five in the pericardium, two in the swim bladder, and one deep in the flesh. In addition to the live postlarvae, eight dead specimens were found attached to the vaiious mesenteries. The dead parasites were small, calcified and brown in colour, except for the proboscides, which remained white.

A postlarva of H. trichuri which had bored through the peritoneum of Macruronus novae-zealandiae was removed from the host and placed in Tyrode's solution. With daily changes of medium the postliava remained alive for 87 days. This appears to be the longest recorded period a cestode has been kept alive in vitro, under non-aseptic conditions.

A detailed account of the anatomy of this species has been given by Dollfus (1942). The proboscides are broad, claviform or rounded, but never long. The proboscide hooks from anterior extremity to tip measure 0.235 mm to 0.250 mm, and the length of the basal insertion is 0.180 mm.

Hepatoxylon megacephalum (Rudolphi, 1819).

(A) Postlarva. Figs. 6–8.

This postlarva was found in the body cavity of Galaeorhinus australis Macleay, from Cook Strait, Notorhynchus pectorosus (Garman), from Dusky Sound, and Dalatius licha (Bonnaterre) from Karkoura. In contracted material when the proboscides are not evaginated, it is difficult to distinguish this species from H. trichiuri.

The proboscides of an extended specimen are conical or cylindrical. The proboscide hooks are in obliquely transverse rows and are the same size for two-thirds the length of the proboscide but become rapidly smaller towards the base. For the larger hooks, the length is 0.140 mm to 0.155 mm with the hooks at the base reduced to 0.035 mm. Smaller hook size together with the shape of the proboscide are the most satisfactory characters for distinguishing this postlarva from that of H. trichiri (Dollfus, 1942).

(B) Adult. Figs. 9–17.

The adult was found in Carcharodon carcharis (Linnaeus) taken in Tory Channel near the breastwork of the whaling station. Four specimens were brought to Wellington in a contracted state preserved in formalin, and the location given as the spiral valve.

The least contracted specimen is 101.5 cm long, and was considerably longer when first removed from the host. Being contracted, the strobila is very broad and thick. At a distance of 2.5 cm behind the scolex, the strobila is 9.0 mm wide and 4.5 mm thick; in the middle region of the strobila the measurements are 22.5 mm wide by 5.5 mm thick The scolex is rounded anteriorly and clearly delimited from the strobila, measuring 8.5 mm long, 9.0 mm broad and 8.2 mm thick. In lateral view the scolex appears oval, with the widest point at the level of the posterior bothridial border. Each bothridial cavity is deep and narrow posteriorly but shallow and wider anteriorly, with the median longitudinal ridge reduced due to contraction.

The proboscides are conspicuous macroscopically and are whitish in colour, compared with the more creamy scolex and strobila. None of the proboscides was completely evaginated. The largest has a length of 1.95 mm. The widest region of the proboscide is close to the base and measured 0.82 mm. The proposcide tapers gradually to a width of 0.40 mm near the distal extremity. The armature is homeo-acanthous—i.e., the hooks are approximately alike in size and shape, and are arranged over the proboscide in about 22 obliquely transverse rows. Proximally the hooks are rudimentary with a very reduced shaft, but they become larger approaching the wides region of the proboscide where they reach maximum size. Maximum length of hook (i.e., from the tip to the anterior extremity), 0.157 mm; length of base of implantation 0.128 mm; distance from point to the posterior extremity 0.065 mm. The anterior extremity of the base of implantation is strongly pointed but is broad and rounded posteriorly.

Proglottids are distinct approximately 9.0 cm behind the scolex. Throughout the length of the strobila the proglottids are always many times broader than long, and strongly craspedote. At the extreme posterior end of the strobila the proglottids split along the mid-line into two segments, each containing a complete set of genitalia.

The surface of the strobila is bounded by a thick cuticle 0.028 mm thick. Immediately internal to the cuticle are found the subcuticular muscle fibres. The longitudinal muscle fibres are arranged in bundles in the cortex near the junction of the cortex and the medulla. Maximum width of bundles 0.042 mm, maximum depth 0.094 mm. Longitudinal bundles are also present in the medulla lateral to the nerve trunks. Internal and external to these bundles are found circular muscle fibres. The longitudinal fibres are continuous throughout the length of the strobila, whereas the circular muscles are not present at the anterior and posterior extremity of each proglottis. Transverse muscle fibres are also found in the cortex extending between the longitudinal bundles into the medulla.

The excretory system consists of four longitudinal canals, arranged in pairs near the lateral margins of the medulla. Each pair consists of an extremely large, thin walled, ventral canal measuring up to 0.358 mm deep by 0.125 mm wide, and a small, rounded, thick-walled, more medial dorsal canal, with a diameter of 0.066 mm. The two ventral canals are joined by a large transverse canal in the posterior region of the proglottis. Throughout the medulla are scattered several (usually 6 to 9) excretory canals, which are anastomosing side branches of the main longitudinal excretory canals.

The nerve trunks occur immediately lateral to the ventral excretory canals in the medulla. They are large, almost circular in cross section with a diameter of 0.082 mm.

The two sets of genitalia in each proglottis are separated except for the vitellaria and testes. Compared with H. trichiuri, the testes are more numerous, and are not distinctly separated into dorsal and ventral fields. They do not extend posterior to the ovary or to the extreme anterior end of the proglottis. The vas deferens is a thin-walled, voluminous, coiled duct, occupying almost the whole depth of the medulla before it enters the vesicula seminalis. At the distal extremity the vas deferens becomes relatively wide and thick-walled, and finally terminates at the vesicula seminalis by a papilla which is lined on both surfaces with cilia. The vesicula seminalis is globose and opens distally into the ejaculatory duct, which coils once then opens into the receptaculum cirri. The internal wall of the invaginated cirrus (which is the external wall when evaginated) is strongly wrinkled.

The vagina is slightly posterior to the ejaculatory duct. Distally it is surrounded by a very thick muscular wall, which is much thicker than that of the ejaculatory duct. A sphincter present in H. trichiuri at the opening of the vagina into the genital atrium is not present in this species. The sphincters are present, however, one at the proximal extremity of the muscular wall of the vagina and a second less