58 PROCEEDINGS OF THE 



way, wliat one might there expect to obtain. Our dredgings, 

 hoAvever, have as yet been too few, and tlie results too incomplete, 

 to enable us to enter into further detail, and to draw out such a 

 list as one would desiderate. 



The Bay of Rothesay is covered with an abundant deposit of 

 soft bluish mud, which few at first sight would be inclined to 

 interfere with ; but on getting to the east of the Old Battery it 

 will be found much harder, with stones and gravel; and upon 

 dredging opposite to this shore, and down towards Huntly Place, 

 among other objects which are generally distributed and usually 

 brought up in every locality, one is pretty sure to obtain several 

 ascidians. Of these the most common was Ascidia mentula, often 

 occurring of large size, and clustered together in great numbers. 

 In many of them the tunic was thickly studded with Crenella mar- 

 morata, set close to each other, and imbedded in its texture. The 

 existence of such a crowd of parasites would lead one to suppose 

 that they cannot be very injurious to the health of the ascidian, 

 and proves the tunic to be very insensible to irritation; were it 

 otherwise, so many large examples would be more rarely met 

 with, and such would be more or less free of them, whereas the 

 size of the ascidian seems only to offer more room for the accommo- 

 dation of the Crenella. Though they may obtain sufficient aliment 

 for themselves, it is possibly liberally increased by a portion of 

 that which the powerful branchial currents of the ascidia brings 

 in for its own use. With these dependants on its outer surface, 

 this ascidia, like Pinna and Modiola modiolus, has occasionally a 

 crustacean residing within its branchial sac, a little entomostracan, 

 NotodelpMs ascidicola. Whether or no the little Pinnotheres ever 

 comes out from between the gills of the Modiola, our first acquaint- 

 ance with Notodelphis was made on observing it emerge at the 

 branchial orifice of this ascidia, and proceed on a tour of inspection 

 over the surface of the tunic. Our desire to lay hold of it, how- 

 ever, was too urgent to wait till we saw whether it would return 

 again within the sac. That its ova are hatched in this situation is 

 highly probable. 



On examining one of the compound ascidians, a species of 

 Amoiiroiiciimi, i:)To\)ab\y A. proliferiwi (common at low water, on 

 stones and in crevices of the rocks on the islets off Millport), and 

 on dissectmg it under the microscope, the individual animals may 

 be readily extracted. From the side of the branchial orifice a 



