PLANARIANS 



79 



external aperture, (2) the secondary branches of these, and 

 (3) the excretory cells with the fine tubules leading from 

 them. Karely is there but a single main excretory trunk 

 present opening at the hinder end of the body (Steno- 

 stoma) ; generally there are a pair of such trunks which 

 open in common at the hinder end of the body, or 

 separately (most Rhabdocoela), or by the mouth (fig. 3). 

 In the Tricladida there are two or 

 four lateral trunks present which 

 open by a number of pores arranged 

 in pairs upon the dorsal surface of 

 the body ; the same appears to be 

 the case in the Polycladida. The 

 main trunks of the excretory sys- 

 tem are generally much twisted in 

 their course, and anastomose with 

 each other ; they receive the fine 

 tubules either directly or, as in the 

 Bhabdocncla, there is a network of 

 secondary tubules interposed. The 

 excretory cells are pear-shaped ; 

 they are branched and furnished 

 with a nucleus and a large vacuole 

 which is directly continuous with 

 the lumen of the tubule ; from the 

 boundary wall of the vacuole springs 

 a single flagellum, which depends 

 into the lumen of the tubule and is 

 capable of active movement. Lang 

 discovered in a marine form of the 

 Tricladida (Otcnda) similar vacuo- i'ig- 3.— Main trunks of the ex- 



,., ,, ' ,1 • 1 n 11 cretory system of -Mesosio^na 



lated ceils with a single flagellum ehrmbergu, o. Sch. open 

 among the epithelial cells of the iVL'^VTlVylT^'' 

 intestine, and came to the conclu- 

 sion that the excretory cells were on that account derived 

 from the epithelium of the intestine. The movements of 

 the excretory fluid towards the external pore are directed 

 by this flagellum as well as by cilia developed upon the 

 walls of the fine tubules ; the motion of all these cilia is 

 such as to drive the contents of the tubules towards the 

 excretory pore. The main trunks of the excretory system 

 are either sparsely (Tricladida according to Jijima) or com- 

 pletely {Polycladida according to Lang) lined with cilia. 



Nerucms System. — The central organ of the nervous 

 system, the brain {en), is a double ganglion at the anterior 

 end of the body, and has been noticed in all the known 

 forms with the exception of the Acoela. It is situated in 

 front of or above the pharynx ; in those species in which 

 a process of the intestine extends beyond the region of the 

 brain {cf. figs. 7 and 8 viewed from the ventral surface) it is 

 placed below this. In such cases there is sometimes a com- 

 missure encircling the prolongations of the intestine. Each 

 of the two ganglia gives o£E a strong longitudinal nerve 

 cord (figs. 5-8, In) from which arise branches going to the 

 various organs of the body. The structure of the nervous 

 system is somewhat different in the Rhabdocoela, Tricla- 

 dida, and Polycladida. In the first group (figs. 5, 6) the 

 two longitudinal cords and their branches are the most 

 feebly developed, and there is but rarely {Mesostoma, 

 Monotus) a transverse commissure uniting the longitudinal 

 cords. These cords are very large in the Tricladida, 

 where the brain is to be regarded as a simple thickening of 

 them ; in this group there are numerous transverse com- 

 missures between the longitudinal nerve cords (fig. 7), and 

 the nerves arising from them and passing to the periphery 

 form a subcutaneous nerve plexus within the muscular 

 coat. Lang has observed a similar nerve plexus in the 

 Polycladida, the central nervous system of which differs 

 from that of the Tricladida in that a number of stout 

 nerve cords radiate outwards from the brain as well as the 



two longitudinal cords; they are all united together by 



-TO 



Fig. 4. Fig. 5. 



Fig. 4. — Plan of an Acoelous Turbellarian. e, eye; m, moutli ; ot, otolitli ; ov, 

 ovary ; p, digesting parenchyma ; t, testiculfir follicles ; vs, vesicula seminalis 

 3 , male organ of copulation ', $ Q , common sexual aperture. 



Fig. 5. — Plan of a Rhabcloccelous Turbellai-ian, 6c, bursa copulatrix ; m, brain ; 

 e, eye ; g, geiTnarium ; «', intestine ; In, longitudinal nerve trunk ; m, mouth ; 

 pit, pharynx ; rs, receptaculum seminis ; s, salivary gland ; t, testis ; «, uterus 

 (containing an egg); v, yelk gland; vs, vesicula seminalis; J, cMtinous 

 copulatory organ ; ^ 9 , common sexual aperture ; be, bursa copulatrix. 



numerous commissures, and a network is thus formed 

 which extends throughout the body. 



Fig. 6. Fig. 7. 



Fig. 6.— Plan of an Alloioccelous Turbellarian. Lettering as in fig. S. 

 Fig. 7. — Plan of a Trieladid. «x, anterior, and tg, is, paired posterior branches of 



intestine ; od, oviduct; te, tentacle ; vd, vas defei'ens ; $ , male, and 9 1 female 



copulatory organ. Other letters as in fig. 5. 



Sense Organs. — These are represented by tactile organs, 



