Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/183

Rh PLANARIANS 173 auditory organs (otoliths), and eyes. The whole surface of the body is very sensitive and (e.g., in the Polydadida} con tains cells which end in tufts of fine hairs, so that certain regions thus become specially sensitive and serve as tactile organs. The anterior pointed extremity of the body in the Rhabdoccela is characterized by an abundant develop ment of rhabdites and tactile hairs, and thus becomes a special tactile organ ; in other cases this region of the body is transformed into a conical tactile proboscis which can be retracted into a sheath (Proboscida). In the freshwater Tridadida the anterior margin of the head is richly inner vated, and is beset with a special row of tactile cells which contain no rhabdites ; in the terrestrial forms of the same family (Bipalium} Moseley has described a row of papillae alon &quot; the crescent-shaped anterior extremity which can be OZA Fir,. 8. Plan of a Polycladid. en, brain; i, intestinal branches; i i, anteri r unpaired intestinal branch; In, longitudinal nerve cord; m, mouth; oil, oviduct; or, ovarian follicle ; pA, pharynx ; p/i t, pharyngeal pouch ; st, stomach; t, testicular folli.-le ; u, uterus; vd, vas deferens; $, male copulatory organ, with the male aperture behind; 9, female copulatory organ, with the female aperture before it. The eyes are omitted. extended and form tactile organs ; between the papillae are peculiar ciliated grooves connected with nerves. In the Polydadida there are tactile cells with stiff hair-like pro cesses on the summit of the dorsal papillae and the various tentacular structures ; the tentacles in this family also serve to support the eyes. The majority of the Turbellarians possess eyes ; the Rhabdocrelidn commonly have two or four, as also have the Tridadid i ; the latter, however, are in some instances furnished with a greater number arranged in a continuous row round the anterior end of the body ; in the Poly- dadida there are from fourteen to several hundred eyes arranged in two symmetrical groups round the brain or scattered over the whole of the anterior margin of the body and upon the tentacles. The eyes are always situ ated beneath the integument within the parenchyma, sometimes directly upon the brain or connected with it by special optic nerves. In its simplest form the eye is a pigmented spot with or without a refractory lens-like body ; the more complicated eyes consist of a pigmented sheath containing a number of refracting rods which are connected at their outer extremity with a series of retinal cells, one to each rod ; the retinal cells are prolonged into a nerve thread running to the brain ; the arrangement of the visual elements is therefore precisely the same as in the vertebrate eye. Of great interest is the fact that in the Polydadida the number of eyes increases with the growth of the animal, and Lang has shown that the eyes increase in number by actual division. On the other hand Carriere has discovered by experimenting with certain freshwater Tricladida that the compound eyes (those containing a number of rods) are formed by the coalescence of several simple eyes. Only a single eye is found in the Jfonotidct, which has the form of a simple pigment spot in front of the otolith. Auditory organs are found in the shape of vesicles filled with fluid and containing circular lenticular or spindle shaped otoliths formed of carbonate of lime. Otolithic vesicles of this kind are found in many Rhabdocoelida (Acoela, Monotida, fig. 4, ot) embedded in a depression on the anterior surface of the brain. In the Dendrocaelida these organs are but rarely present. As a sensory organ of unknown function must be men tioned the paired lateral ciliated grooves which are met with on either side of the brain in many RhaMo (da (fig. 9, c) ; they are also found commonly in NEMERTINES (q. v.), but are here more complicated in structure. Reproductive Organs. With a few exceptions all the Turbellarians are hermaphrodite, and reproduce themselves sexually. Only among the Microstomida is there an asexual as well as a sexual reproduction. The male and female organs open to the exterior, either through a common cloaca (atrium genitale) on the ventral .sur face (most Rhabdoccelida and all Tricladida, figs. 4-7), or there are separate male and female apertures. In this case the male aperture is generally placed in front of the female aperture (some Rhabdu- ccelida and all Polydadida, fig. 8), but occasionally the positions are reversed (certain RhaMocaelida). The genital glands display a primitive condition in being paired, though frequently the ger- inarium (fig. 5, g) of the Rhabdocaila, and occasionally also the testis, is developed only upon one side of the body. The structure of the female organs varies. In some cases there are simple ovaries (ov in tigs. 4, 8) in which the ova originate and become fully mature without being furnished with the secretion of a second gland ; in other cases there is a division into ger- marium (fig. 5-7, g) and yelk gland (v) ; the primordial ova or germs originate in the former, and absorb the products of the yelk gland in the atrium, where they become ready for fertilization. An intermediate condition is seen in those forms where there is but a i simple gland present which produces germs in its upper portion j and yelk in the lower portion. The ovaries are generally compact I round or tubular glands (fig. 4); sometimes they are formed of a I number of pear-shaped follicles (fig. 8) ; there is usually a simple or paired uterus (u) which retains the ova for some time before they are deposited ; sometimes, however, the ova undergo their develop ment within the uterus and are completely developed before expul sion ; in some cases the egg-shell is detached within the uterus so that the young are produced alive. In Turbellarians without a yelk gland the uterus is a simple widening of the oviduct (fig. 8^; in those forms which possess addi tional yelk glands the uterus is a simple or paired diverticulum of the atrium genitale (figs. 5, 7). The ova are either surrounded by ! a more or less hard chitinous shell, or one shell contains a number I of ova (&quot;cocoon&quot; of Tricladida and many Polycladida). The ! Polydadida deposit an egg-string which like that of the Gastropoda ! consists of a number of eggs bound together by a transparent 1 albumen-like mass. Many Rhabdocosl Turbellarians (e.g., Mesostoma chrenbergii) produce two sorts of ova, thin-shelled summer ova and i thick-shelled winter ova ; the latter are capable of withstanding a considerable amount of desiccation, and are deposited in the autumn. The accessory female organs of reproduction are represented by bursa- seminales, which receive the semen during copulation and retain it