196 LIVEKPOOL BIOLOGICAL SOCIETY. 



Order I. MYXOSPONGIiE. 



Halisarca rubra, n. sp. (PL X., figs. 1 and 2.) 



New species of Halisarca have been described so fre- 

 quently, which have afterwards been shown not to belong- 

 to that genus or even not to exist at all, that it is with 

 some reluctance that I establish the new species Halisarca 

 rubra. The specimen was dredged on the " Spindrift" 

 Expedition in July, 1889, off Holyhead, from a depth of 

 about fifty fathoms. It encrusted both valves of a living 

 Mytilus edulis with thin brick-red patches, the entire thick- 

 ness of the sponge being 0'45 mm. Its surface showed a 

 somewhat wavy outline, which condition was apparently 

 solely caused by the hairs of the Mytilus projecting through 

 it, and the sponge growing for a short distance upwards 

 along those hairs. Oscula and pores were not visible to 

 the unaided eye. 



Vertical sections showed that the outer portion of the 

 sponge had suffered, so that its structure could not be made 

 out satisfactorily. The figure (see PL X., fig. 1) of it 

 therefore is somewhat diagrammatic. The inner and 

 greater portion of the sponge was well preserved (PL X., 

 fig. 2). There is a " dermal membrane " between the outer 

 world and subdermal cavities, about 0*014 mm. in thickness. 

 The subdermal cavities are flat, and seem to be distinct 

 from the wide irregular cavities of the canal system. 

 Oscula and pores could not be detected. The flagellated 

 chambers are round or oval, with a diameter of 0'08 to 

 0"14 mm. The size of the collar-cells, of which however 

 the collars and flagellae were never seen distinctly, is about 

 0*006 mm. The mesoderm consists of fibrous tissue. Im- 

 bedded in it are large red pigment-cells, 0*02 to 0'026 mm. 

 in size, more or less oval and pretty numerous. Their 

 nuclei are small, and sometimes only indistinctly seen. 



