NARRATIVE OF THE CRUISE. 



281 



of the most characteristic peculiarities of the Echinoderms 

 in general, and doubtless plays a much greater role in their 

 life. 



" With regard to the sensory organs of the Elasi- 

 poda, it is very remarkable that auditory organs in the 

 shape of small sacs, with a greater or smaller number of 

 otoliths, are often very abundantly developed, while no 

 traces of eyes are apparent, and that these organs are 

 in connection not only with the nerve ring but also 

 with the two lateral ventral nerve stems. Possibly 

 the well-developed auditory organs, together with the 

 tactile organs, viz., the dorsal processes, can to some 

 extent supply the want of eyes. With good reason it may 

 be asked why many species are so richly provided with 

 auditory organs — some species have fifty sacs or more — 

 while other forms are totally devoid of them. As is well 

 known, only a few shallow water Holothurians are furnished 

 with auditory vesicles, but these have always their place at 

 the nerve-ring. 



" In the Elasipoda, as in the majority of the Holothu- 

 rioidea, the sexes are distinct. The embryo of the 

 Echinoderms commonly leaves the egg in a condition very 

 different from the adult state, and their larvae live under 

 conditions totally different from those under which the 

 adult passes its existence. Thus, while the adult animal 

 moves slowly along the floor of the bottom, the larvae are 

 found living on the surface of the sea, carried about by 

 winds and currents, until they have reached that stage in 

 their evolution, when it becomes necessary for their exist- 

 ence and further development to retire to the shores or 

 the bottom of the sea. In some Echinoderms the embryo 

 passes into the adult condition without any free larval 

 stage, which seems to be the case with several shallow 

 water Holothurians, and doubtless even with the Elasi- 

 poda. If these latter were subject to a more complicated 

 metamorphosis, it is most probable that their larvae would 

 not be able to live at the surface of the sea, but keep close 

 to the floor of the ocean. It may be stated here that 

 during the Challenger Expedition some particularly intcr- 



(.NAKR. CHALL. EXP. — VOL. I. — 1884.) 



A 



o 



Fig. 112.— Psychropotes 



Tli.VI. 



