Sphaeropleaceas 273 



branched, the latter simple and completely free. Only a few ' aplanospores ' 

 are produced and they are calcified. 



The genera are : Acetabularia Lamouroux, 1816 [inclus. Polyphysa Lamarck, 1816 ; 

 and =Acetabulum Lamarck (vide Borgesen, '08)]; Acicularia D'Archiac, 1843; Halicoryne 

 Harvey, 1859 ; Chalmasia Solms-Laubach, 1895. 



Halicoryne is a link with the other sub-families of the Dasycladacese. Except for 

 Acetabularia mediterranea all the representatives of the sub-family are tropical or 

 subtropical, occurring mostly in shallow water, and often fastened to shells and stones. 

 Species of Acetabularia are frequent in the mangrove-formation of the West Indies. 



Family Sphaeropleacese. 



This family includes but one genus Spht&roplea which occupies a some- 

 what isolated position in the Siphonocladiales. The filaments of Sph&roplea 

 are cylindrical and unbranched, consisting of series of greatly elongated 

 coenocytes. They are free-floating and never possess any organs of attach- 

 ment. The transverse walls are sometimes of great thickness, whereas 

 the side walls of the coenocytes are comparatively thin. In rare instances 

 the transverse walls are not fully closed owing to the imperfect development 

 of the ingrowing ring-like septa. In the vegetative coenocytes there is a 

 thin lining layer of protoplasm in which there are apparently annular green 

 chloroplasts separated by broad colourless bands (fig. 176 A). Each annular 

 band is in reality a close aggregate of small parietal chloroplasts not 

 essentially different from those present in other members of the Siphono- 

 cladiales ; the aggregation is, however, so dense that the individual chloro- 

 plasts are scarcely discernible. Some of the larger chloroplasts (from four to 

 six in each ring) contain a single pyrenoid, but the remainder are without 

 pyrenoids. In the colourless zones between the annular aggregations of 

 chloroplasts the lining layer of protoplasm is very thin, but in the coloured 

 zones the protoplasm is much concentrated and often extends right across 

 the lumen of the segment as a sort of diaphragm or plug. It is in these 

 regions that the nuclei are situated (fig. 176 A n), generally just within the 

 parietal chloroplasts. A few chloroplasts sometimes extend from ring to 

 ring along strands of the protoplasm. 



Asexual reproduction is unknown, but there is a sexual reproduction of a 

 relatively high type.' The sexual organs are antheridia and oogonia, formed 

 without change of shape from any segment of the filament. Sometimes the 

 oogonia and antheridia alternate, but more often they do not. In the forma- 

 tion of the antherozoids there is a repeated mitotic division of the nuclei, 

 accompanied by a disappearance of the pyrenoids and a division of the 

 chloroplasts, the latter ultimately becoming pale yellow in colour. Each 

 antherozoid is a minute, elongated, spindle-shaped body, with a small 



W. A. 18 



