Sub-Fam. 2. Phyllacanthinae. Gen. _Calliobothrium_, _Anthobothrium_, etc. 5. Fam. TAENIIDAE. Sub-Fam. 1. Cystotaeninae. Gen. _Taenia_ s. str. Sub-Fam. 2. Anoplocephalinae. Gen. _Moniezia_, _Thysanosoma_, _Stilesia_, _Anoplocephala_. Sub-Fam. 3. Cystoidotaeninae. Gen. _Dipylidium_, _Hymenolepis_, _Drepanidotaenia_, _Dicranotaenia_, _Echinocotyle_, _Davainea_. Sub-Fam. 4. Mesocestoidinae. Gen. _Mesocestoides_, _Dithyridium_. Sub-Fam. 5. Ichthyotaeninae. Gen. _Ichthyotaenia_, _Corallobothrium_. {92}CHAPTER IV MESOZOA DICYEMIDAE—STRUCTURE—REPRODUCTION—OCCURRENCE: ORTHONECTIDAE— OCCURRENCE—STRUCTURE: TRICHOPLAX: SALINELLA. The Mesozoa are an obscure group, the position of which in the animal kingdom is still doubtful. The name Mesozoa was given to the group by its discoverer, E. van Beneden,[112] as he concluded that they were intermediate between the Protozoa and the higher Invertebrates. Recent authors, however, have called attention to the resemblance existing between them and the "sporocysts" of Trematodes, and though we still are ignorant of certain important points in their life-histories, the Mesozoa are most conveniently (and probably rightly) considered as an appendix to the Platyhelminthes. [Illustration: FIG. 45.—A, B, C, Stages in the development of the vermiform larva in _Dicyema typus_ van Ben. (After Ed. van Beneden.) _cal_, "Calotte"; _gc_, germinal cell; _n_, nucleus of endodermal cell.] The animals composing this group are minute and parasitic, and are composed of a small number of cells. They may be divided into two families: the _Dicyemidae_, which occur exclusively in the kidneys of certain Cephalopods (cuttle-fish); and the _Orthonectidae_, which live in the brittle-star _Amphiura squamata_, the Nemertine _Nemertes lacteus_, or the {93}Polyclad _Leptoplana tremellaris_. In addition to the undoubted Mesozoa, certain anomalous forms—_Trichoplax adhaerens_ and _Salinella salve_—may be referred to this group. [Illustration: FIG. 46.—_Dicyemennea eledones_ Wag., from the kidney of _Eledone moschata_. A, Full-grown Rhombogen with infusoriform embryos (_emb_); B, one of the latter developing; C, fully formed; D, calotte, composed of the upper nine cells shown in the figure. (After Ed. van Beneden and Whitman.) _emb_, Infusoriform embryo; _g_, part of endoderm-cell where formation of these embryos is rapidly proceeding; _n.ect_, nucleus of ectoderm-cell; _n.end_, nucleus of endoderm-cell; _p_, "calotte."]