134 SUMMARY OF CURRENT RESEARCirES RELATING TO 



nnd M. Lcidey'i were foiiml, while slcntlcr waving processes of 

 S. lacushin, totally colourless, could be seen reaching up througli tho 

 mud in little groups upon the bottom. Mr. Potts had always held 

 that it was imjiossible for sponges to live upon a muddy bottom, and 

 theoretical reasoning would still suggest that probably only this species 

 which can thus hold itself up out of the suffocating silt can survive 

 the constant deposition of siliceous particles. 



The commensal habit of many of the lower animals who feed by 

 the creation of ciliaiy whirlpool currents, has been frequently referred 

 to ; the weaker current-makers, such as VoiilecUce, Stcntors, and tho 

 errant and tubicolous rotifers, planting themselves about the heads of 

 tho stronger Bryozoa to supply their own nets with what may have 

 escaped from the others. The same instinctive principle which leads 

 all these to locate themselves most ])leutifully amongst the stones in tho 

 rapids of streams, was particularly noticeable in promoting their 

 aggregation upon and in the neighbourhood of the inlet and outlet 

 gates of the reservoirs. The feeble currents produced by each can 

 only bring within its reach the floating provision from a very limited 

 area ; the volume of water i^ourcd through these gates brings to thera 

 a rich sujiply, and the numbers and variety of these organisms 

 increase in proportion. Of the fixed forms there were seen amongst 

 the Bryozoa — beside one or more undetermined species oi Plmnatella — 

 PeciinateUa magnified, and TJrnatella gracilis of Leidy, and the newly 

 described PaludiccUa erecta. Attached to these were Vorlicelhv, 

 £pistifJis, and Stcntors innumerable ; Pyxicola and Acineta ; rotifers of 

 various names, incluJiug prominently Limnias and other probably 

 undcscribcd forms among the Melicertidte. Very abundant among 

 these was the interesting chastobranch annelid Mnnayunkia speciosa 

 Leidy, and the hydroid Cordi/lophora lacustris. This last was parti- 

 cularly abundant around the south-east outlet ; its stems forming a 

 complete matting over many yards of surface, commingled with 

 Bryozoa and sponges in intricate confusion. 



In a spot where all light was absent was found an incrustation of 

 3/8 in. in thickness, composed of gemmules and spicules of Mei/euia 

 Leidyi with stems of PhimateUa, Urnatella, and Curdijlophora. The 

 fact that these can thus thrive in absolute darkness throws some doubt 

 on the sujiposed sensitiveness of the forms to the presence or absence 

 of light. The new species Paludicella erecta was found in the glare 

 of full sunlight, while P. Ehrenhergi is said to seek the darkest 

 corners. 



Mollusca. 



Radula of Cephalophorous Molluscs.* — Dr. R. Rossler comes to 

 the Conclusion that the seat of formation of the radula of all Molluscs 

 is to be sought for in the papilla which traverses the hinder wall of 

 the pharynx on its lower surface and projects by its end into the 

 body-c.ivity. This radula-pouch is a diverticulum of the oral epi- 

 thelium, the lumen of which is filled by a mass of connective tissue, 

 which functions as a supporting and nutrient''apparatus. The radula 



* Zeitschr. f. Wiss. Zool., xli. (1S85) pp. 447-82 (2 pis.). 



