1058 



MR. CYRIL CROSSLAND ON 



critical stage is that during which the free-swimming larva must 

 attach itself to a suitable substratum or perish). At this second 

 stage the Aviculidas at least are in the most active state they 

 attain to, and most other normally sessile lamellibranchs have 

 well-developed and actively vised feet. Tridacna gigas is veiy 

 active at this stage, to mention a form probably not often seen 

 when so young, and particulai'ly well fixed down when adult. 

 Their future life depends upon their success in crawling into the 

 position adapted to the special needs of their species. Also they 

 are now large enough to be attractive as food to fishes and crabs, 

 but not strong enough to resist claws and pincers. In the case 

 of M. margay'itifera, among other species, the animal has little 

 to fear after it is one year old, being then too strong-shelled for 

 ordinary fishes and crabs. 



Text-fia-. 230. 



Two specimens of ^yicwZa zehra attached to Millepora alcicornis, and in the centre 

 of the latter a specimen of another genus of the Aviculidaj which is more 

 common on coiuls. Another species of Avicula is found on " bhick coral," 

 Antipatharia. Half natural size. 



Batistes flavimarginatus and B. viridescens, the largest of a 

 genus of shell-eating fish, are fairly frequently found here * in the 

 Red Sea, and occasionally break up " pearl " oysters two or three 

 years old, but those dissected were found to have preferred smaller 

 and weaker-shelled species. The big Rays are not common, and 



* Dongonah, Port Sudan. 



