INVEUTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 469 



what oval, baviug merely the histological value of a single simjile cell), 

 encloses a nucleus or inner vesicle of large size, usually more than 

 half the diameter of the central capsule, and sometimes with one 

 large nucleolus and sometimes several. The cell-membrane is always 

 double, pierced by one or more large openings, through which the 

 intra-capsular protoplasm communicates with the much more abun- 

 dant cxtra-capsular protoplasm. In the latter, towards the outside, 

 lies the Phfeodium, a peculiar thick mass of dark pigment-granules 

 (or Pha3odella), which are usually dun brown or black-brown, often 

 greenish or dun green and like the Phseodium of varying form and 

 size. Sometimes the Phfeodium envelojies the greater part of the 

 capsule, sometimes only one side of it. The whole body is enclosed 

 in a thick gelatiuous covering, which is often provided with spaces 

 which the numerous pseudopodia traverse in order to radiate freely 

 beyond its outer surface. With very few exceptions (the small divi- 

 sion Phfeodinidese) a well-developed, always extra-capsular siliceous 

 skeleton is secreted which forms, as in the different groups of the 

 typical l\adiolaria, very varied and delicate structures usually radia- 

 ting outwards in hollow siliceous tubes. 



According to the structure of the siliceous skeleton Professor 

 Haeckel distinguishes in the grouj) four orders and ten families. 



Order I. Phaeocystia. — The siliceous skeleton is either entirely 

 wanting or it consists of hollow spines, arranged sometimes irregu- 

 larly, sometimes regularly, outside the central capsule. 



Family 1. Pha3odinid8e. — Siliceous skeleton entirely wanting. 

 Genera : Phceodina, Phceocolla. 



Family 2. Cannorhaphidfe. — The siliceous skeleton consists of 

 numerous separate hollow sjiines, or portions of hollow network, 

 which, scattered round the periphery of the extra-capsular soft 

 substance, are usually arranged taugentially. Genera : Cannorhaphis, 

 T]i((lass(>plancta, Didyocha. 



Family 3. Aulacanthida3. — The siliceous skeleton consists of 

 hollow radial sinnes, which sjiring from the outer surface of tho 

 central capsule, and traverse the extra-capsular jolly. The outer 

 surface of the jelly is usually covered by a thick mantle of line 

 hollow siliceous needles, which are arranged tangentially and felted 

 together. Genera : Aulacantha, Aulancora, Aulograpliium. 



Order II. Phajogromia. — The siliceous skeleton consists of a 

 • single fenestrated shell wliich is of different forms, sometimes round, 

 sometimes egg-shaped, often dipleuric, but always furnished with a 

 large principal opening or mouth (more rarely with several openings). 

 Hollow spines with pe<;uliar i)orc-areas at their bases are often 

 present. 



Family 4. Challengeridic. — Tlie siliceous skeleton consists of a 

 fenestrated slKdl, uniaxial or dii)leuric, often laterally compressed and 

 carinated, often egg-shaped or oval, and furnished with a wide open- 

 ing at one end of the axis. This mouth is seldom simple, it is usually 

 armed with a hollow tooth, or with one or mon; often branched 

 hollow tubes. The fenestrated structure of tho siliceous shell re- 

 sembles most closely that of the diatoms; tlu^re is a ihio pore in the 



