Genus. Dibothriocephalus, Lühe, 1899.
Syn.: Diphyllobothrium, Cobbold, 1858; Bothriocephalus, p. p. Rud., 1819; Dibothrius, p. p. Rud., 1819; Dibothrium, p. p. Dies., 1850.
Scolex egg-shaped; dorsal and ventral bothria elongated, moderately strong, cutting rather deeply into the head; genitalia single in each proglottis; papillæ in the vicinity of the genital atrium; the testes and vitellaria are in the lateral fields, the former in the medullary layer, the latter in the cortical layer on both surfaces, and occasionally extending to the median line; the ovary ventral, the shell gland dorsal. The uterus is in the central field, taking a zigzag course, and frequently forms a rosette.
Dibothriocephalus latus, L., 1748.
Syn.: Tænia lata, L., 1748; Tænia vulgaris, L., 1748; Tænia grisea, Pallas, 1796; Tænia membranacea, Pall., 1781; Tænia tenella, Pall., 1781; Tænia dentata, Batsch, 1786; Bothriocephalus latus, Bremser, 1819; Dibothrium latum, Dies., 1850; Bothriocephalus cristatus, Davaine, 1874[281]; Bothriocephalus balticus, Kchnmstr., 1855; Bothriocephalus latissimus, Bugn., 1886.
Length 2 to 9 m. or more; colour yellowish-grey; after lying in water the lateral areas become brownish and the uterine rosette brown. The head is almond-shaped, 2 to 3 mm. in length, the dorso-ventral axis is longer than the transverse diameter; the head, therefore, generally lying flat, conceals the suctorial grooves at the borders; these suckers are deep and have sharp edges (fig. 205). The neck varies in length according to the degree of contraction and is very thin; there are 3,000 to 4,200 proglottids and there may be more; their breadth is usually greater than their length, but in the posterior third of the body they are almost square, and the very oldest are not uncommonly longer than they are broad. There are numerous testes situated dorsally in the medullary layer of the lateral fields; the vas deferens (fig. [192]) passes dorsally in transverse loops in the central field anteriorly and forms a seminal vesicle before its entry into the large cirrus pouch.
The orifice of the vagina is close behind the orifice of the cirrus; the former passes almost straight along the median line posteriorly, and widens into a receptaculum seminis shortly before its junction with the oviduct; the ovary is bilobed, in shape like the wings of a butterfly, ventrally in the medullary layer; the shell glands lie in the posterior recess of the ovary; the uterus, forming numerous transverse convolutions, passes ventral to the vas deferens forwards. Eggs (fig. [207]) large, with brownish shells and small lids, 68 µ to 71 µ by 45 µ; the ovarian cell, which is already, as a rule, in process of segmentation, is surrounded by numerous large yolk cells; the proglottids nearest the posterior extremity are frequently eggless.
Fig. 204.—Various chains of segments of Dibothriocephalus latus, showing the central uterine rosette. (Natural size.)