74 ] The Classification of Lower Organisms 



numerous little naked protoplasts. These protoplasts are said to bear flagella; whether 

 one or two, equal or unequal, is not certainly known. They are supposed to escape 

 and function as zoospores, but Karsten (1904), on rather scant evidence, supposed 

 them to be gametes. 



A protoplast may contract and form a shell within its former shell. The new shell 

 consists like the old one of two parts, one fitting within the other. The outer shell is 

 usually more or less elaborately sculptured, while the inner is smooth. It is supposed 

 that the outer shell is deposited between outer and inner masses of protoplasm, and 

 that the entire protoplast then withdraws to the interior and deposits the inner shell 

 in the opening. It is in this manner that the statospores of chrysomonads are formed. 

 The resting cells of diatoms as just described are believed to be homologous with 

 them, and are called by the same term. 



As a third manner of producing a reproductive cell, a protoplast may expand, force 

 apart the valves of its shell, and deposit an enlarged shell about itself. The resulting 

 spore is called an auxospore. As noted, Iyengar and Subrahmanyan found the pro- 

 duction of auxospores in Cyclotclla to involve sexual processes. 



Schiitt divided the Centricae into three groups with names in -oideae (presum- 

 ably subfamilies) and these into nine groups with names in -eae (presumably tribes). 

 Subsequent authorities have made of Schiitt's groups a varying number of families. 

 The minimum tenable number of families is three, corresponding to Schutt's 

 subfamilies. 



Family 1. Coscinodiscea [Coscinodisceae] Kiitzing Phyc. Germ. 112 (1845). 

 Family Melosireae Kiitzing op. cit. 66. Families Melosiraceae and Coscinodiscaceae 

 West British Freshw. Alg. 274, 276 (1904). Melosira, in fresh water, the shells feebly 

 silicified, the cells joined end to end in filaments. Cyclotclla, separate drum-shaped 

 cells in fresh water. Coscinodiscus, the cells disk-shaped. Triceratium, cells of the 

 form of 3-, 4-, or 5-sided prisms with abbreviated axes. 



Family 2. Rhizosoleniacea [Rhizosoleniaceae] West British Freshw. Alg. 278 

 (1904). The cells, circular or elliptic in cross section, becoming elongate by inter- 

 calation of ring-shaped bands of wall between the valves. Rhizosolenia. Corethron. 



Family 3. Biddulphiea [Biddulphieae] Kutzing Phyc. Germ. 115 (1845). Families 

 Biddulphiaceae and Chaetoceraceae Auctt. Cells laterally compressed, elliptic in 

 valve view, oblong or rhombic in girdle view. Cells of Biddulphia, solitary or colonial, 

 are familar as epiphytes on marine algae. Chaetoceros, the cells with a long spine at 

 each corner, frequently united valve to valve in filaments, abundant in subpolar 

 oceans. 



Order 2. Diatomea [Diatomeae] C. Agardh Syst. Alg. xii (1824). 



Tribe Striatae with orders Astomaticae and Stomaticae, and tribe Vittatae also 



with orders Astomaticae and Stomaticae, Kutzing Phyc. Germ. ( 1845). 

 Pennatae Schiitt in Engler and Prantl Nat. Pflanzenfam. I Teil, Abt. lb: 101 



(1896). 

 Order Pennatae Campbell Univ. Textb. Bot. 90 ( 1902). 

 Order Naviculales Bessey in Univ. Nebraska Studies 7: 284 ( 1907). 

 Diatoms basically of isobilateral symmetry, occasionally so skewed as to be dorsi- 

 ventral or asymmetric; valves usually punctured by a longitudinal cleft called the 

 raphe, or bearing a marking of some sort, called the pseudoraphe, in the same posi- 

 tion; exhibiting, when possessed of a true raphe, a gliding motion; cells usually with 

 two plastids. 



