MOLUULA SOCIALIS. 57 



small base. Apertures terminal, approximate, rather 

 small, tubular. Text greenish, thin, soft, covered with 

 longish, unbranched, rather rugged, glandular fibrils. 

 Mantle greenish, soft, attached to the test only at the 

 tubes. Tentacular filaments large, much branched, 

 tripinnate. Bra urinal sac with six folds on each side; 

 the meshes very short, irregular, and imperfectly con- 

 voluted. Oral la/niita very narrow. Intestine forming 

 two loops. Reproductive organs narrow, elongated, 

 slightly arched. Densely gregarious. 



Height about half an inch. 



Halt. [Coralline zone (?), attached to shells.] 



ENGLAND. --The "Diamond' trawling ground near 

 Hastings, Sussex [attached to Pecten maximus~\ (Boioer- 

 bank). 



: 



FIG. 44. Part of branchial sac of Molgula socialis. Much enlarged. 



First record. Alder, 18(53 ; coll. Bowerbank. 



Unlike the other species of the genus, which are 

 generally solitary, this Molgula is associated in dense 

 masses, firmly adhering to each other by the instru- 

 mentality of the fibrils, and so closely crowded are 

 they as frequently to press the sides, giving rise to a 

 square, pentagonal, or hexagonal form. The test is 

 touo-h and densely covered with fine sand. 



The mantle (PL XXVII, figs. 1 and 2), though 

 transparent, is thicker than usual and is pretty- 

 uniformly of a pale green colour; the radiating muscles 

 at the base of the tubes are delicate and rather incon- 

 spicuous. 



The external surface of the branchial sac (PL XXVIII, 



