MINTAD FROM IfOETH-WEST AUSTRALIA. 115 



Description of the Specimen. 



External Foo-m.—T\\e body is rounded, flattened above ; 

 marked by twenty grooves wbicli extend from near the apex to 

 the edges of the oral disk ; they are fainter neal- tlie disk than 

 elsewhere in the wall of the body. Tlie integument between each 

 groove is marked by transverse lines, which do not always extend 

 from groove to groove, and so present a gyriform rather than a 

 sulcate appearance. The apex of the air-chamber is exactly at 

 the aboral pole, and though the body-walls are strongly contracted 

 it is quite apparent ; the circular sphincter w^hich surrounds it is 

 marked off by a circular groove from the body-wall, and has an 

 extraordinarily close resemblance to the periproct of a regular 

 echiuid. 



Colour greenish brown. 



Measurements. — Diameter 15, height 10, diameter of oral 

 disk 4'2, diameter of orifice of air-chamber 1*5, diameter of 

 circular sphincter 4 millim. 



Mouth deeply withdrawn. 



Tentacles. — Numerous, short, simple ; no perforation to be 

 detected at their tip ; apparently dicyclic, but the appearance 

 may be due only to crowding. 



Mesenterial Septa twenty ; corresj)onding to the grooves of the 

 wall. 



Air-chamler spacious, not communicating with the gastric 

 cavity or its annexes ; not hollow, but containing a body of gela- 

 tinous appearance in spirit, which is found under the microscope 

 to be formed of fine fibres of connective substance. 



Zoological Affinities of the S^oecimen. 

 The latest definition of the group to which the specimen 

 belongs is that of Dr. Andres; but as he merely refers to the 

 habit of swimming freely on the surface, we must go back to that 

 of Milne-Edwards and Haime, who group together all such 

 Actinidae as have the pedal disk purse-shaped under the head of 

 the Minyadinse ; these may have the tentacles smooth or com- 

 posite, and the former may have the integument verrucosa or 

 smooth. It is in the last division that the specimen now under 

 study will have to be placed. The only genus now in the division 

 is that of Plotactis, which is thus defined by its authors (Milne- 

 Edwards and Haime): — " Tentacules simples et allonges; corps 



