THE FORMATION OP THE ORGANS THE SENSORY ORGANS. 199 



and becomes a vesicle which, however, remains open (Fig. 90 B}. Its 

 lumen is filled with a strongly refractive gelatinous mass (gl) known 

 as the vitreous body. In other forms the vesicle has closed (C) 

 and, finally, the higher form of Gastropod eye (D) provided with 

 a so-called lens and vitreous body is developed (FRAISSE, No. 34 ; 

 HILGER, No. 43 ; PELSENEER, No. 85). [In most Diotocardia the 

 optic vesicle is open, but in the specialised Helicinidae and 

 Ncritidae (the Gymnopoda of FISCHER) and in the Turbinidae it is 

 closed as in all tne Monotocardia.] 



The first-named Prosobranchs are held on other grounds to be 

 primitive forms, and the simple structure of the eye seems therefore 

 probably a primitive condition. If this supposition is correct, we 

 should here see with special clearness the gradual development of 

 the optic organ up to its present level. 



According to CARRIERE (No. 22), in cases where the eyes are regenerated, 

 their formation takes place in the same way as when they arise ontogene- 

 tically. The ectodermal epithelium is thus at a later time also capable of 

 giving rise to the sensory organs. 



The otocysts, which are specially distinct in the larva, and the 

 origin of which has already been alluded to several times (Figs. 55, 

 59, 65, 72, 79), appear as depressions of the ectoderm on either side 

 of the pedal rudiment, near the pedal ganglion, with which, however, 

 they do not come into any closer relation as they are innervated from 

 the brain (LACAZE-DuTHiERs). When cut off from the ectoderm, 

 these walls are still formed of long cylindrical cells which flatten 

 later ; but, for a time, the anterior and ventral part of the vesicles 

 still remain thick. From this part of the wall, the otolith or otoliths 

 (otoconia) are secreted ; these structures become detached from the 

 wall and rest upon the sensory hairs which have arisen on the 

 cells. 



Spengel's (olfactory) organ (osphradium) only develops at a later 

 stage (Paludina). It arises as an ectodermal thickening composed 

 of several layers of cells. Where, as in Paludiua, pits are found in 

 the organ, these are caused by depressions in the thickened ectoderm 

 (v. ERLANGER). 



The pectinate condition of this organ, which is found in many Gastropods, 

 arises in a similar way. The organ was originally paired and lay near 

 the gill, as may still be the case in Zygobranchiate Diotocardia. Where 

 it is single, as in the Monotocardia and the Euthyneura, this is in all cases 

 connected with the asymmetry caused by the torsion of the visceral mass. 



