PHYSIOLOGY OF THE INVERTEnRATA. 351 



refracting cones. Subjacent to these, Professor V. Vl. 

 Duncan, F.R.S.,* finds ganglionic cells and nerve-plexuses. 

 It would seem, therefore, that these bodies are rudimentary 

 eyes." 



The colouring matter of the blue ocular spot of the 

 above-mentioned species of ^r^/?j<a has been spectroscopically 

 investigated by MacMunn,t and these investigations have led 

 him to believe that it is possible that this pigment is capable 

 of absorbing certain rays of light, so as to enable the animal 

 to distinsruish liojlit from darkness. 



The Echixodermata. 



The sense of touch is well developed in the Ecltiiiodcrmata, 

 and seems to have its seat in the ambulacral feet, pedi- 

 cellaria?, and tentacula situated in the neighbourhood of the 

 buccal orifice. Homanes and Ewart state that " all the 

 Echinodcrnuda seek to escape from injury. Thus, for in- 

 stance, if a starfish or sea-urchin is advancing continuously 

 in one direction, and if it be pricked or cut in any part of an 

 excitable surface facing the direction of advance, the animal 

 immediately reverses that direction." There is no doubt that 

 the sense of touch is present in these animals. 



The sense of smell also appears to be developed to a certain 

 extent in starfishes. If several of these animals (contained 

 in a tank) are advancing in the direction of a luminous 

 portion of the water.:}: they immediately retract their steps, if 

 a small quantity of bromine or sulphuretted hydrogen water 

 is gently poured into the luminous portion of the water. 

 This fact appears to support the idea of a sense of smell in 

 the Askridca. 



According to Leydig§ certain Echinodcrmata appear to be 

 provided with auditory vesicles, in which float powerfully 



* Proceediiu/s of Boyal Society, 1873. 



t PhiloKophical Transactions, 1885, pt. 2, p. 660. 



t The water was illuminated bj' means of a small incandescent lamp. 



§ Histologic Comparce, p. 316, 



