BAJALTJS. 31 



plane. The oscula are situated terminally on the processes, so that there are 

 as many vents to the sponge as there ai'e branches. The oscula are surrounded 

 by small " chimneys " similar to the tubes described by Schulze, which appear 

 as prolongations of the osculum-margin in Oscarella lobularis. The surface of 

 the sponge is perfectly smooth, as in Halisarca dujardini. The oscula measure 

 from 1 to 2 millim. in width, and are liable to great alterations in size. The 

 "chimney" can be retracted, so as to leave the oscular opening nearly bare. 

 The inhalant pores measure 0*1 millim. across ; they are circular. Each is 

 covered by a thin and tender perforated plate. The perforations are circular 

 or polygonal, with rounded corners, and measure 0*01 millim. in diameter. 

 These little pores are liable to great alterations in size, and can be contracted 

 and even closed by the sponge. 



Canal-sijstem. — The outer skin is divided from the interior of the sponge — ■ 

 from the zone of ciliated chambers — by a broad aubdermal cavity 0-15 millim. 

 wide. This cavity is continuous ; it is traversed in all directions by a highly 

 complicated network of fine threads, measuring 0*005-0"01 millim. in thick- 

 ness. These repeatedly ramified, anastomosing threads are cylindrical, and 

 between the joining-points more or less straight. They connect the skin and 

 the body of the sponge ; and appear to be to a certain extent contractile. 

 The zone of ciliated chambers is folded. Broad and conic inhalant canals 

 lie between the folds. 



The ciliated cliamhcrs are of a regular elongate, oval, cylindrical shape. They 

 are longer than in Aplysilla, and somewhat similar to the radial tubes of the 

 Syconidaj or the ciliated chambers of Euplectetta. They measure 0'17 millim. 

 in length, and are 1 millim- wide. The afferent pores form groups of three 

 to five. Some chambers are in direct communication with the subdermal 

 cavity ; whilst others draw their supply of water from the inhalant canals. 

 The ciliated chambers are not constricted at their exhalant aperture, which 

 is circular and opens either into a narrow eshalant canal, or direct into the 

 gastral cavity in the centre of the sponge. 



The exhalant canal-system consists of narrow more or less radial canals, 

 which extend slightly upwards towards the osculum ; these canals are cylindrical, 

 and curved in such a manner that their distal portion runs for a short distance 

 parallel to the outer surface, whilst their proximal part is radial, and often 

 stands at nearly a right angle to the former. These canals open out into an 

 extensive gastral cavity which occupies the central portion of the sponge. This 

 cavity is traversed by a few stout ramified threads of tissue. The narrow 

 exhalant branch canals have a diameter of O'l to 0-3 millim. The central 

 gastral cavity has a diameter equal to a fourth or a third the diameter of 

 the part of the sponge in which it is situated. The threads pervading it in 

 varying direction are distant and rare ; they are more or less cylindrical, and 

 measure 0*1 millim. in thickness. Towards the osculum they become more 



