276 DEsCEiFTiON OF {^Pulygastrka. 



Genus CLOSTErvruM. — Frond simjjlc, elongated, lunutely curved or 

 arcuate, entire; the junction of the segments marked by a pale 

 transverse band, but no constriction. 



In the usual position, the opposite margins of the fusiform fronds 

 are always more or less dissimilar, the upper one being convex, and 

 the lower, inclusive of the ends, straight, or more commonly concave ; 

 the lateral view is straight, and has both margins similar. As happens, 

 in the allied genera, the frond is composed of two segments which 

 finally separate, and allow the escape of the endochrome. The seg- 

 ments are entire, without spines, processes, or inflated protuberances. 



The endochi'ome is green, with darker longitudinal bands or fillets, 

 the number of which varies in different sjiecies. The diaphanous 

 vesicles, conspicuous in most of the species, are either scattered, or 

 arranged, with more or less regularity, in a single longitudinal series. 



At each extremity of the endochrome, even in its earliest state, 

 there is a large hyaline, or straw-coloured globule, which contains 

 minute granules in constant motion. This globule disappears in 

 the dried specimen. 



A distinct circulation (rotation) has been noticed in several species. 

 It is said to occur only in species obtained from water, and not in 

 those taken from moist ground. The empty frond is striated in some 

 species, and smooth in others. The colour' and firmness of the cover- 

 ing differs in different species. Some are quite coloiu-less, flexible, 

 collapsing when dried, and in general allowing the endochrome to 

 escape by a merely partial separation of the segments. These species 

 are never striated. In other species the fronds arc more or less straw- 

 coloured, or even reddish. The deeper the colour, the fii'mer the 

 fi'ond. The segments in such, separate entu-ely from each other. 

 Some of the striated species, even when submitted to the action of 

 nitric acid or fire, retain theu- form and markings. In the coloured 

 species the extremities are generally darker than the rest of the 

 frond." 



Even in the firmest species the frond becomes flattened in drjdng, 

 its breadth at the centre increases, and the ends appear more 

 attenuated than in the living state ; a fact to be borne in mind in 

 describing or drawing a dried specimen. 



In Closterium the process of conjugation appears to be nearly the 



