132 INVERTEBRATE MORPHOLOGY. 



and containing an otolith of carbonate of lime, is present in 

 some of the lower Turbellaria, as Monotus, and rests directly 

 upon the surface of the brain ; these structures probably, as 

 in the Cnidaria, are sense-organs of equilibrium rather than 

 of audition. In the Polyclads tentacles are frequently pres- 

 ent, sometimes capable of being retracted and serving as 

 organs of touch, and in certain Rhabdoccels there is a ciliated 

 depression on each side of the head richly supplied with 

 nerves forming what has been considered an olfactory organ. 



1. Order Acoela. 



The Acoela form a group of lowly-organized Turbellaria 

 exclusively marine in habitat and leading an active and free 

 existence. They all possess a mouth (Fig. 71, ni) situated on 

 the ventral surface and leading into a short pharynx, though 

 in some forms this may be absent; but beyond this there is 

 no trace of a digestive tract, the food passing from the 

 pharynx into the parenchyma (p), where it is digested. Ow- 

 ing to the lack of a digestive tract these forms are strictly 

 two-layered (Fig. 69, A), only the ectoderm and mes-endoderm 

 being represented, and consequently are exceedingly interest- 

 ing as indicating the manner in which the differentiation of 

 the triploblastic condition has been derived from the diplo- 

 blastic. 



The nervous system has been described in Convoluta as 

 consisting of a bilobed ganglion surrounding the otocyst, and 

 in front of this and united to it by commissures is a second 

 pair of ganglia. From the anterior ganglia there arise by a 

 common stem two nerves on each side which pass backwards, 

 one along the edge of the body and the other a little internal 

 to it, while the posterior gangliouic mass gives rise to two 

 nerves which pass backwards, one on each side of the median 

 line. All six nerves send ofl' numerous transverse branches 

 which unite to form with the nerve-cords a square-meshed 

 network. In addition to the single otocyst (Fig. 71, ot] two 

 pigment-spots lying in the ectoderm and representing light- 

 percipient organs (e) are present, as well as a peculiar refrac- 

 tive highly-movable organ, lying in the median line on the 



