EARLY DEVELOPMENT OF GRIBRELLA OGULATA. 



413 



right half of the anterior coelom, as found in the unpaired condition (in which case it 

 would fuse with the left anterior ccelom later in development), then it is evident that a 

 median organ arising from the anterior ccelom, like the central ccelom, must in Echinus 

 arise either from the right or the left half of the anterior coelom, and as it moves to the 

 right side after origin in Cribrella, we should expect to find it originating from this 

 side in Echinus. After origin it appears to move into a median dorsal position on the 

 oesophagus (m.v., fig. 3), where it has been noticed in pelagic larvae by Bury. 



Table showing relationships of Ccelomic Elements in Various Types. 



Balanoglossus. 



Cribrella Larva. 



Asterid. 



Crinoid. 



Holothurian. 



Proboscis cavity. 

 Left collar-cavity. 

 Right collar-cavity. 

 Trunk cavities. 



Pre-oral coelom. 



Left lateral coelom. 



Right lateral coelom. 



Posterior coelom 

 (L + R). 



Axial sinus, etc. 

 Hydroccele. 

 Epigastric coelom. 

 Hypogastric coelom. 



Parietal canal. 

 Hydroccele. 

 Aboral coelom. 

 Oral ccelom. 



(Vestigial.) 

 Hydroccele. 

 Right body-cavity. 

 Left body-cavity. 













[6. — Diagram 



TRANS. ROY. SOC. EDIN., VOL. XL. PART II. (NO. 19). 



3 Q 



