80 



PLANARIANS 



auditory organs (otoliths), and eyes. The whole surface of 

 the body is very sensitive and (e.ff., 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 Rhabdocaila 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 papillte 

 along the crescent-shaped anterior extremity which can be 



ov- 



extended and form tactile organs ; between the papilke are 

 peculiar ciliated grooves connected with nerves. In the 

 Piilydndidn there are tactile cells with stiff hair-like pro- 

 cesses on the summit of the dorsal papilla; and the various 

 tentacular structures ; the tentacles in this family also 

 serve to support the eyes. 



The majority of the Turbellarians possess eyes ; the 

 Rhabdocoelida commonly have two or four, as also have the 

 Tridndidn ; 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- 

 cladida 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 

 Polycladida 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 

 Tridadida 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 Monotida, 

 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 Dendrocoelida 

 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 Rhabdoccela (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 com- 

 mon cloaca (atrium genitale) on the ventral surface (most 

 Rhabdocoelida and all Tridadida, 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 Rhabdocoelida and all Polycladida, fig. 8), but 

 occasionally the positions are reversed (certain Rhabdo- 

 coelida). The genital glands display a primitive condition 

 in being paired, though frequently the gerrnarium (fig. 5, 

 </) of the Rhabdoceela, 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 figs. 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 germarium (fig. 5-7, y) and 

 yelk gland (c) ; 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 simple gland present which produces germs 

 in its upper portion and yelk in the lower portion. The 

 ovaries are generally compact round or tubular glands 

 (fig. 4) ; sometimes they are formed of a 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 development within the uterus and are completely 

 developed before expulsion ; in some cases the egg-shell is 

 detached within the uterus so that the young are produced 

 alive. 



In Turbellariuns without a yelk gland the uterus is a 



