Evolution and Symmetrij in the Order of the Sea-pens. 117 



example, the autozooids are distributed all over the rachis except along a 

 short and narrow groove on one side. 



In Sclerobelemnon there is usually a well-marked dorsal track free from 

 autozooids, but in some specimens one or, more rarely, more than one 

 autozooid occurs in the course of the dorsal track. And in this genus there 

 is no trace of a ventral track. In Anthoptiluiu there is always a complete 

 dorsal track but usually very little trace of a ventral track. And finally in 

 Pteroeides, Pennatula and some other genera both tracks are always present 

 and always free from autozooids. 



Deeply imbedded in the tissues of the rachis and stalk there is usually 

 found a long calcified rod, known as the axis. It is always present in 

 bilaterally symmetrical genera, but it may be absent or incomplete in some of 

 the radially symmetrical genera. 



In the walls of the zooids and in the surface tissues of the rachis and stalk 

 there are usually found numerous calcareous spicules. The shape and size 

 of these spicules are usually regarded as of the greatest importance in the 

 diagnosis of genera and species, but although they are of great value as 

 accessory characters in recognising some forms, and particularly in the 

 bilaterally symmetrical genera, they are so variable, both in form and size, in 

 others that they may be most unrealiable for systematic work. 



It is unfortunate that owing to the deep-sea habits of the sea-pens we know 

 very little about their powers of movement and habit. Eumphius,* in 1705, 

 stated that the Sagitta marina aJha (probably a species of Virgularia), found 

 at low tide on the shores of Amboyna, burrows deeper and deeper into the 

 sand as the tide ebbs, and Darwin relates that the sea-pens (probably a species 

 of Stylatula) found off the coast of Patagonia, " when touched or pulled, 

 suddenly drew themselves in with great force, so as nearly or quite to 

 disappear." The huge OstcoceUa septentrionalis of British Columbia is said to 

 writhe like a worm when it is caught, and there are scattered observations 

 that in an aquarium Pteroeides will bore with its stalk into the sand and 

 draw itself upright. But these observations were all made on bilaterally 

 symmetrical sea-pens and we are still without information, by direct 

 observation, about the activities and habits of the radially symmetrical 

 forms. 



We know then from direct observation that the sea-pens, unlike other 

 Alcyonarian colonies, can move through the sand in which they burrow and 

 that in some cases these movements are rapid and powerful and therefore 

 must be due to muscular contraction. 



We do not know from observation that the movements of the bilaterally 

 * Rumpliius, ' D'Amboinsche Rariteitkamer,' 1705. 



