REPORT ON THE RADIOLARIA. 721 



radiate within a spherical space (equally disposed iu all directions), but withiu a 

 quadrant or even an octant, forming a conical brush or pencil. 



One very remarkable form of A c t i n e 1 i d a is Actinastrum, forming the transition 

 from these Adelacantha to the common regular Icosacantha. In the two observed 

 species of Actinastrum we find thirty-two radial spines, twenty of which are disposed 

 after the Miillerian law, as in the Icosacantha. The other twelve are four interradial 

 equatorial sjjines (lying in the two secondary meridian planes) and eight perradial 

 tropical spines (Ipng in the two primary meridian planes). Therefore here in each 

 primary meridian plane are placed ten spines (two equatorial, four tropical, and four 

 polar spines), whereas in each secondary meridian plane are placed six spines (two 

 equatorial and four tropical). But here also all thirty-two spines are so regularly placed 

 that their free distal ends fall into five parallel zones, four in each polar zone, eight in 

 each tropical zone, and eight in the equatorial zone. 



Tlie Central Junction of the radial spines in the Acantharia becomes efi"ected in 

 four different ways : — (l) by simple apposition of the pyramidal central ends or bases ; 

 (2) by a basal leaf-cross, or by broad wings, four on each spine, supported one upon the 

 other ; (3) by a central concrescence of the meeting bases of aU the twenty spines, growing 

 perfectly together; and (4) by a concrescence in pairs of every two opposite spines. The 

 most common and probably the original mode of junction is the first — by pyramidal 

 apposition ; the spines at the central base are pointed in the form of a pj^ramid, and the 

 triangular faces of the neighbouring pyramids are simply placed upon one another. 

 Often the small basal pyramids are imperfectly separated from the spines by an annular 

 constriction. Commonly the basal pjTamids of the four equatorial spines are six-sided, 

 those of the sixteen other spines five-sided. 



The second mode of junction, by a basal leaf-cross, is developed from the first and 

 appears as a strengthening or a mechanical elaboration of it. Immediately above the 

 basal pyramid arise from its radial edges four thin and broad triangular leaves or wings, 

 and the meeting edges of the neighbouring wings are in apposition one with the other, 

 so that between the bases of every three or four neighbouring spines a hoUow 

 pyramidal space remains oj^en. The apex of such a pyramidal space is directed towards 

 the centre of the body, but separated from it by the small basal pyramid ; its 

 open base is directed outwards. The twenty-two hollow pyramidal spaces are 

 disposed regularly in four different groups : — (A) Four equatorial spaces, four-sided, 

 each limited by two equatorial and two tropical spines (one canceral and one capricornal) ; 

 (B) eight perizonal spaces (four northern and four southern), four-sided, each limited 

 by one ecpiatorial, two tropical, and one polar spine ; (C) eight peripolar spaces (four 

 northern and four southern), three-sided, each limited by one tropical and two polar 

 spines ; (D) two polar spaces (one northern and one southern), four-sided, each limited 

 by four neighbouring polar spines. 



(ZOOL. CHALL. ESP. — PART XL. — 1885.) Rl" 91 



