Japanese Tricladida Marioola. 2-7, 



Sensory Organs. 



The organs considered to be sensory organs are the tentacles 

 or the corresponding anterior parts which are devoid of rhabdites 

 as well as pigments and the eyes. 



Tentacles. — The antero-laterally directed appendages projecting 

 from the frontal margin are generally shown as tentacles, which 

 are found only in Pr. lactea among the species examined by me. 

 Unlike Pr. ohlinl, the epidermis of this part is composed of 

 normal ciliated cells which ahvays contain no rhabdites and pig- 

 ments. Ill St. trlgonocephala also the epidermis lias the same 

 appearance at the anterior end of the body ; while in Kct. limuli 

 it is wholly devoid of nuclei which have sunk into the paren- 

 chyma just beneath tlie sheet of the dermal musculature. The 

 minute structure of the cells has already been described in the 

 part dealing with epidermis. No peculiar sensory cell has come 

 under my observation at all. 



Eyes. — Generally two eyes lie on either side of the median 

 line on the dorsal surface in the anterior region. They are em- 

 bedded in the parenchyma. In Pr. lactea the eyes are situated at 

 a little distance from the brain mass, but in St. Irigonoce'phala and 

 Ect. limidi either near or close to it. They are usually of an 

 ovoid or crescentic shape. So far as my observations go, the 

 structure shows no differenee in essential respects from that 

 observed in Pr. ulvae by Hesse (2(S). We can distinguish two 

 elements of which a single eye is formed, viz., the pigment cup 

 and inside the pigment cup the retina cell (PL I., fig. 4, re). 

 The pigment cup is composed of only a single celb which shows a 

 crescentic appearance, as recorded by Hesse. On the lateral side 

 of it there exists an ellipsoidal, homogeneous or finely granular 

 plasmic lens layer (/). This, in Pr. lactea and St.. triganocephala, 

 is developed to a considerable extent. Tlie nucleus does not 

 occur in the lens layer, but lying just outside the pigment layer, 

 as is shown in fig. 4 (n). 



In structural respests the retina cell has very nearly the 

 same appearance as that previously observed üi several 



