228 Siphonales 



found albuminous bodies similar to those which occur in Bryopsis, and also 

 crystals of calcium oxalate. 



Reproduction occurs only by zoogonidia which are produced in sub- 

 spherical or club-shaped zoogonidangia borne laterally on the erect branches 

 (fig. 147 1 and 4)- The zoogonidangium, which is separated from the main 

 axis by a thick, lamellate, basal wall, and often by a small basal cell, produces 

 from 8 to 20 zoogonidia. These are comparatively large, ovoid in shape, 

 and furnished with a circlet of cilia at the anterior end (fig. 147 5). 



The protoplasm within the zoogonidangium contains at first several thousand nuclei of 

 slightly larger size than the chloroplasts. Soon a process of nuclear differentiation sets 

 in, certain of the nuclei increasing to about four or six times their original size. These 

 rapidly acquire a definite cytoplasmic envelope, with radiating strands, and become 

 distributed uniformly through the zoogonidangium. The smaller nuclei soon degenerate ; 

 they gradually decrease in size until they are much smaller than the chloroplasts, lose 

 their chromatin, and finally disintegrate in the cytoplasm. When this nuclear degenera- 

 tion has practically ended the segmentation of the protoplasm takes place, cleavage 

 beginning at the periphery. The developing zoogonidia become rounded and the numerous 

 chloroplasts arrange themselves radially. The nucleus then moves from the centre 

 towards the periphery, and on the side of the nucleus nearest the periphery some of the 

 protoplasmic strands arrange themselves in the form of a funnel and numerous granules 

 which occur on these strands move outwards towards the plasma-membrane. These 

 granules accumulate in a circle just underneath the plasma-membrane and fuse with one 

 another to form a deeply-staining, firm ring, which is the blepharoplast. The proto- 

 plasmic strands connecting the nucleus with the developing blepharoplast apparently 

 disappear after its formation, and the nucleus passes back to the centre of the zoogonidium, 

 the chloroplasts once more arranging themselves radially. The blepharoplast then splits 

 to form two rings, one slightly below the other, and the circle of cilia is developed from 

 the lower ring (Davis, '08). 



In the possession of a circlet of cilia the zoogonidia of Derbesia resemble 

 those of (Edogonium, but this resemblance does not in any way suggest 

 affinity. The zoogonidia of Derbesia are formed from a multinucleate 

 segment in which the nuclear changes are unusual, and their peculiarities 

 may be entirely the result of the exceptional manner of their origin. The 

 investigations of Davis ('08) indicate that it is in the highest degree 

 probable that the ancestors of Derbesia produced numerous biciliated 

 zoogonidia similar to those which are at present produced in many genera 

 of the Siphonales, and that the peculiar zoogonidium, like that of Vaucheria, 

 is only a late phylogenetic development. It is not unlikely that Derbesia 

 may be distantly related to Vaucheria. 



Family Caulerpaceae. 



This is another family of the Siphonales represented by a single genus 

 Caiderpa Lamouroux, 1809 of which about 60 species are known from 

 tropical and subtropical seas. This genus is one of the most singular types 



