37° Copepoda living as Messmates with Ascidians. [Sess. 



in each single Ascidian. The specimens were of a narrow 

 cylindrical form, and the ovisacs they carried were slender 

 and elongated, being about twice the length of the entire 

 animal. They had a general resemblance to Enterocola, and 

 were at first ascribed to that genus. 1 A more familiar ac- 

 quaintance with these interesting Ascidian Copepoda, however, 

 showed that they could not retain their position in van Beneden's 

 genus, and that their true place was with the Aplostoma of 

 Canu. They are closely allied to Aplostoma brevicauda Canu, 

 but appear to differ in some minor details of structure : they 

 have therefore been recorded under the name of Aplostoma 

 affinis? Dr Canu has obtained A. brevicauda in Morchellium 

 argus M.-Edw., Amarcecium Nordmanni M.-Edw., and Poly- 

 clinum luteum Giard. 



Genus Lichomolgus Thorell. 



Notwithstanding that there are numerous Copepoda in- 

 cluded in the Cyclopoida, and also though not a few of 

 them have been recorded as the semi-parasites or messmates 

 of various other groups of invertebrates, scarcely half a dozen 

 species — and all of them limited to the genus Lichomolgus, as 

 that genus is at present denned — have been found among the 

 different forms associated with the Tunicata. 



Lichomolgus forficula Thorell appears to be the most common 

 and generally distributed species, and it is scarcely ever found 

 anywhere else than in the branchial sac of the larger Ascidians. 

 This Lichomolgus is readily distinguished from its nearest 

 allies by the two long tail - appendages : both of them are 

 jointed near the middle, and this in itself is an innovation 

 on what is usual among Copepoda. The species is small and 

 of a whitish colour, and it is active in its movements, running 

 with considerable agility over the surface of the branchial sac. 

 I have found it moderately common in large Ascidians 

 dredged in Scapa Flow, Orkney, and in the Clyde, but there 



1 " On some New or Rare Crustacea from the Firth of Forth," ' Ann. and Mag. 

 Nat. Hist.' (6), vol. x. p. 203, pi. xvi., figs. 1-11 (1872). 



- "Catalogue of Forth Crustacea," by T. Scott, 'Proc. Roy. Phys. Soc.,' vol. 

 xvi. p. 363 (1906). 



