874 



EMBRYOLOGY 



the Echinodermata, and it was indeed supposed tliat the 

 acorn represented a I'udiment of this system, especially since 

 the cavity of the acorn in its earliest condition presents a 

 certain resemblance to the water- vascular system as it origi- 

 nates in the Echinodermata. It appears certain that the 

 acorn serves as an organ of locomotion. It was "believed that 

 it took in water from the outside by means of the proboscis 



pore, and that there- 

 fore it operated in the 

 same way as the am- 

 bulacral feet of the 

 Echinoderms (Spen- 

 Gel). However, it has 

 been maintained, on 

 the other hand, that 

 particles of pigment 

 distributed in the water 

 are never found inside 

 the acorn, and that the 

 proboscis pore therefore 

 does not serve for the 

 reception, but only for 

 the elimination, of sub- 

 stances from the inside 

 (Bateson). This ob- 

 servation is of parti- 

 cular interest,inasrauch 

 as the acorn contains 

 a glandular structure, 

 which has been inter- 

 preted as an excretory 

 organ. The locomotion 

 of Balanoglossus is 

 effected by" peristaltic 

 movements on the part 

 of the acorn, which thus pushes its way into the sand. The 

 collar follows it, also taking part in the same way in the pro- 

 gression ©f tb.e animal. At the same time the sand enters 

 •the mouth-opening, which lies at the base of the acorn, 





dlj. 



J 



Fig. ICl. — Balanoglossus Ko[ralcv:ikii (after 

 A. Agassiz). e, acorn; !cr, collar; fc, branchial 

 region; g, genital region of the body; c'6, 

 dorsal, vh, ventral, blood-vessel. 



