214 PLATYHELMIXTHES. 



animals. No doubt what we really have to do with here is a con- 

 tinuous system of tubular A r acuoles in the plasmatic parenchyma- 

 similar in nature to the tubes leading from the contractile vacuole 

 of an Infusorian. 



The excretory system has not yet been observed in the Acoela. 



The nervous system (Fig. 168) consists of a bilobed ganglion at 

 the anterior end of the body. It is embedded in the parenchyma, 

 and gives off nerve trunks in all directions ; of these, two especially 

 large lateral trunks run backwards, one on either side (Fig. 168). 

 The latter are sometimes connected at regular intervals by delicate 

 transverse coids, and in the Polychula and Trt<-h~i<la all the peripheral 

 nerve-trunks anastomose. In a number of the two last-named groups 

 a diverticulum of the enteron runs forward above the transverse 

 commissure in a groove between the two cerebral lobes ; and in some 

 genera this enteric prolongation is encircled by a nervous commissure. 



Sense-organs. Eye-spots are very generally present. In the lower 

 forms (Acoela} they are simply pigment-spots in the ectoderm. As 

 a rule, however, they are placed in the parenchyma or in the brain, 

 and consist of many retinal rods contained in a pigmented sheath. 

 The outer ends of these rods, i.e., the ends turned towards the 

 external surface, are continuous with the nerves passing to the brain. 



In the Polyclaila the eyes have been observed to increase by 

 division, and in the Tridaila they have been seen to fuse, so as 

 to give rise to more complex eyes. The eyes, which may be present 

 in any number, from one to a hundred or more, are placed over the 

 brain, or on the tentacles, or on the margin of the body. 



Auditory organs, as otocysts, are sometimes found. In Acoela 

 and in some RhaMocoela (Monotus), and more rarely in Polydada, 

 a single otocyst is -found in the neighbourhood of the brain. 



The integument is endowed with a highly developed tactile sense ; 

 the large hairs and stiff bristles which project between the cilia have 

 probably this function. Lateral ciliated pits, which may also be 

 explained as sense-organs, are in rare cases present at the anterior 

 end of the body. In the Prolo$ci<lae the anterior end of the body 

 is retractile into a sheath, and probably highly sensitive. 



Reproductive organs. With the exception of Microstomwn and 

 Stenostomum, and possibly Plagiostomum cUoicum, the Turljellaria are 

 hermaphrodite ; but steps between the hermaphrodite and dioecious 

 condition are not wanting, for according to Metschnikoff in Gy rotor 

 liermaphroditus (Prostomum lineare), the male generative organs are 

 sometimes developed, while the female organs remain rudimentary, 



