82 



THE PINEAL ORGAN 



sometimes phosphorescent, or luminous in the dark, and are arranged 

 round the margin of the umbrella. They are found on the outer side of a 

 hollow, club-shaped process called the tentaculo-cyst (Fig. 52, B, C), 

 which lies between each pair of marginal lappets. These marginal lappets, 

 of which there are eight, are situated between the tentacles around the 

 edge of the umbrella (Fig. 52, A, B). They lie in relation with the outer 

 end of each " radial " and " per-radial " canal and between each pair is 

 a prolongation of the " circular " canal, namely the tentaculo-cyst. 



BRAIN & SUPRA- 

 OESOPHAGEAL GANGLION 



PAIRED 

 •-^•-r* LATERAL 

 NERVES 



WW 



SUB-EPIDERMALNERVE NET W MEDIAN NERVE-CORD 



ABC 



Fig. 53. — Diagram showing the Origin of the Central Nervous System 

 and Primary Nerve Trunks from a Sub-epidermal Nerve Net. 



A — Low type of flat worm ; B — Higher type (Planaria) ; C — Arthropod. 



(After Jijima Hatschek, and Stempel.) 



Another type of sense-organ which must be considered along with the 

 ocelli is the " statocyst." These frequently contain calcareous particles 

 which are supported on cilia and constitute one of the earliest types of 

 equilibrating organs, such as are represented in vertebrates by the paired 

 systems of semicircular canals and vestibules of the internal ear. Such 



circ. c. : circular canal. 



gp. : gastric pouch. 



ir. c. : interradial canal. 



h. : hood. 



/. .- lithite. 



mg. I. : marginal lappet. 



mth. : mouth. 



oc. : eye-spot. 



olf. \ olf.' 1 : olfactory pits. 



or. a. : oral arms. 



st. : stomach. 



t. : tentacles. 



tc. : tentaculo-cyst. 



(After Lankester, from A Textbook of Zoology, Parker and Haswell.) 



