1920.1 NATURAL SCIENCES OF PHILADELPHIA. 207 



NOTES ON ARACHNOIDISCUS. 

 BY SARAH P. MONKS. 



While studying specimens of Hemiqjtychus (Arachnoidiscus) , which 

 are extremely abundant in many San Pedro, California, tidepools, 

 an unusual form was found which may be called a variety of Arach- 

 7ioidisciis ehrenbergii . 



Instead of being circular ^\^th raj^s of equal length, it is blunt- 

 cuneiform, with sides compressed and two sets of short rays. In 

 all but shape and rays, even in various sizes and deformations, it 

 is a true Arachnoidiscus, and I have called it A. ehrenbergii var. 

 cuneatus. 



It is quite abundant, thirty specimens being found in thirty micro- 

 mounts. 



The change of shape is no doubt partly due to overcrowding, for 

 although there are miles of tide-pools and millions of sea-plants to 

 choose from, the diatoms are often on some plants in almost in- 

 credible abundance. 



These ahen epiphytes crowd on many different algae — on stiff 

 Gelidium, stony Corallines, and even on the flexible stems of other 

 plants. Sometimes in shallow tidepools nearer land which are 

 exposed to wdnter cold, or summer heat, during very low tide, the 

 diatoms are killed, and then the host plant is gray-coated like sleet- 

 crusted trees in winter. But when alive the brown of the diatoms 

 entirely covers the stem of the host 'like a shiny varnish, and the 

 only chance the burdened alga has is for terminal growth. When 

 the brown film dies, on exposure to the sun or the dry air, the whole 

 colony shows the green of chlorophyll, and this green remains for 

 years; then when nothing remains but skeletons the effect is gray- 

 ish white and the diatoms still stick to the host plant. Not only 

 are the algae burdened with "an innumerable host" of Arachnoidis- 

 cus, but there are co-dwellers, members of ten or more other genera. 



The habits of these diatoms may account for the many and vari- 

 ous irregularities of Arachnoidiscus. Species of Isthmia hang in 

 festoons and swing away from the alga's stems^ as do some Biddulpha 



