TRICLAD TURBEI.LARIA FROM INDIAN TIBET / 



Polycelis tibetica n. sp. 



Syti. Sorocclis sp. Meixner und Mutli, 1911. 



1. Extcrm^l features. As only preserved specimens are available, the size and shape of 

 the living animal cannot be stated. In preserved specimens there is always a certain amount 

 of distortion. The size appears to be moderate, probably not exceeding 15 mm. Judged 

 from the least distorted specimens, the head has much the same shape as in Polycelis coronata 

 (see Hyman, 1931a). The anterior margin (Plate I, fig. 5) is rounded in a gentle curve 

 and provided laterally with two fairly prominent auricles.^ The arrangement of the eyes is 

 also similar to that of P. coronata, consisting of a semicircular band passing near the anterior 

 margin, along the base of the auricles, and terminating shortly behind the auricles. Behind 

 the auricles, the body incurves slightly, then broadens toward the middle regions, and finally 

 again diminishes towards the rounded posterior end. Plate I, fig. 1, gives the appearance of 

 one of the less contracted specimens. Presumably in life the auricles would be more promi- 

 nent, probably still more extended, than in the specimen shown in Plate I, fig. 5, and the body 

 somewhat more elongated. As is common in the genus, the pharynx is very long and 

 powerful, terminating near the posterior end and leaving only a short region for the sexual 

 apparatus. 



The eyes in all the specimens form a semicircular band several eyes wide running along 

 the margin of the head and terminating" shortly behind the auricles. The number and 

 arrangement of the eyes vary in different specimens, no two worms being exactly alike in 

 these respects. Frequently the posterior end of the band is at a different level on the two 

 sides. The number of eyes varies definitely with age, consisting of thirty to forty in the 

 smallest specimens, sixty to eighty in medium specimens, and eighty to one hundred or more 

 in the largest worms. Plate I, figs. 2 to 5, inclusive, indicate this relationship with age. In 

 Plate I, fig. 6, where the number of eyes is quite small although the worm was a large one, 

 there were various indications that the head was in process of regeneration. 



The coloration appears to be a uniform dark brown or black. 



2. General histological features. Some of the specimens are in a good state of fixation 

 but the histology presents nothing in particular. The ventral epithelium is cuboidal and is 

 ciliated throughout. The dorsal epithelium is slightly taller, contains more rhabdites, and is 

 ciliated at least in part. The anterior part of the dorsal surface is always ciliated to a greater 

 or less extent while posterior dorsal regions usually lack cilia. The adhesive zone is very 

 narrow but so far as could be determined follows the usual course completely encircling the 

 body margin. The most conspicuous histological elements are the lar;^e gland cells found in 

 the anterior half of the body distributed thickly below the intestine, slightly less abundant 

 dorsal to the intestine. They consist chiefly of the large rounded gland cells taking basic 

 stains which are called Eiweisszellen 1)y German authors. Gland cells of this tyjie and location 

 are very common in planarians. 



The pharynx has the structure typical of the family Planariidae, the circular and longi- 

 tudinal muscle layers of the inner muscular zone being distinctly separated into two strata. 



3. Reproductive system. This system follows the plan typical of the genus Polycelis. 

 Of the four sexual specimens found in the collection two came from station K 78 and two 



'In Plate I, fig. la, I have ventured to add a copy of a sketcli of the anterior end of a living animal, made 

 from one of the specimens of the series from K 66. — G. E. H. 



