STRUCTURE AND DEVELOPMENT OF CESTODA.[105]—Of the unsegmented Cestodes, _Caryophyllaeus mutabilis_, from the intestine of carp and other Cyprinoid fishes, is the most easily accessible form. _Triaenophorus nodulosus_, which is very useful for the study of the excretory system, occurs mature in the pike. In the body-cavity of the Stickleback (Fig. 40) a large, broad, yellow worm may sometimes {85}be found, the larva of _Schistocephalus solidus_ Crepl., which occurs in the intestine of Terns, Storks, Mergansers, and other birds. Species of _Ligula_ are found in the same birds. The intestine of a _Lophius_ or _Cyclopterus_ ("lump-fish") contains, usually, the early and intermediate stages of various Cestodes, while the alimentary canal of Elasmobranchs often contain many peculiar Tetrarhynchidae and other forms. For the study of development, the _Taenia anatina_ from the duck may be used. The ripe proglottides are collected, and the eggs placed with _Cypris ovum_ in an aquarium, with the probability that some of the embryos will enter the Ostracod, and the peculiar Cysticercoid may be bred.[106] _Cysticercus pisiformis_ and _Coenurus serialis_, which occur commonly in rabbits, are also suitable objects for examination. A Cestode such as _Echinobothrium_ (Fig. 36) is divisible into head and proglottides. Moniez has suggested that the head is really the morphologically hinder end of the body, in which case the formation of proglottides would closely resemble the mode of segmentation of an Annelid larva. The close similarity, however, between the Cysticercoid larva (Fig. 43, F) and the Cercaria of a liver-fluke, seems to show that the anterior end is the same in both cases, and since it bears the central part of the nervous system, we may reasonably call it the "head." Moreover the hinder end of a Platyhelminth usually possesses the chief excretory pore. Another difficulty is the determination of dorsal and ventral surfaces. Authors are agreed,—on the analogy of Trematodes, in which the testes are usually dorsal and the ovaries ventral,—that the dorsal and ventral aspects of a Cestode are determined by the position of these organs, although the often radially formed "head," the lateral or superficial position of the genital apertures, and the variability of these features, render it a matter of considerable doubt whether "dorsal" and "ventral" are more than useful conventional terms. The suckers and hooks are borne on a muscular cap, the "rostellum," which is only slightly developed in the _Ichthyotaeniae_. The body is solid, and is divisible into an outer muscular coat—enveloped in a (possibly epidermal) investing membrane—and an inner parenchymatous tissue containing the chief part of the excretory, nervous, and reproductive systems. One or two pairs {86}of longitudinal excretory vessels are present, usually connected by transverse ducts and opening by a single terminal pore. Occasionally a regularly paired arrangement of lateral or