52 . THE AMERICAN MONTHLY [March, 



denly expanded into a ball, while at the other it is attached to a bit of 

 water-weed, or other foreign object, as, for instance, the shell of another 

 individual of its own kind (Fig. i). 



" Examined under a higher (?;-inch) power, the stalk is seen to be 

 an empty tube, as is proved by its doubly-contoured margins in optic 

 longitudinal section, divided or frayed out at its base into two or more 

 root-like processes for attachment, whilst the ball becomes resolved 

 into a regular or slightly elongated hollow sphere, perforated on all 

 sides by large holes separated from one another by bars or bridges of 

 the substance forming its wall, much in fact like a piece of pei'forated 

 zinc, but with these differences, that while in perforated zinc the holes 

 are invariably of the same size and shape, and are invariably separated 

 by bars of equal width throughout, in the shell of Clathrnlina they 

 often vary in size from specimen to specimen, and are hence separated 

 from one another by bars of vai'ying width — these being sometimes 

 narrow and rod-like, and sometimes wide and plate-like — and they also 

 differ in shape, varying from round or oval to polygonal with the angles 

 of the polygon rounded off. The sphere and the stalk, which consist 

 from first to last, of a single continuous piece, form a skeleton; the 

 sphere serving as a supporting and protective envelope or shell for the 

 body of a delicate unicellular animal allied to the well-known sun 

 animalcule {Actinophrys sol) , and the stalk furnishing a firm though 

 flexible support for the shell and its occupant. The skeleton, which is 

 siliceous, is colorless in young specimens, and sometimes remains so 

 throughout life, but more commonly assumes with age a yellowish or 

 brownish colpr, which in full grown examples is so deep as greatly 

 to interfere with the observation of the structure of the body of the 

 heliozoan. The external surface of the shell is marked by a net-work 

 of grooves, which are sometimes so faintly indicated as to be scarcely 

 traceable, but at others are very deep, in which latter case their margins 

 are so prominent as in optic ti^ansverse section of the shell to present 

 the appearance of spines. 



"The stalk also varies greatly in length, ranging from twice to six 

 times the diameter of the shell. Of recorded measurements the max- 

 ima are: Diameter of shell, .072 mm. ; diameter of stalk, .003 mm. ; 

 length of stalk, 3.000 mm. 



" The protoplasmic body of the animal may in young specimens 

 completely fill or even envelop the shell ; but in fully developed and 

 active individuals it does not nearly do so, a wide space intervening 

 between its outer surface and the inner surface of the shell. It exhibits 

 no differentiation into exoplasm and endoplasm. In outline it is irreg- 

 ular, its surface being drawn out into lobes here and there at the origin 

 of numerous radiating pseudopodia, in which respect itoflersa marked 

 contrast to Actinophrys^ whose contours are regular. Two kinds of 

 vacuoles are present in the granular protoplasm of which the animal's 

 body is composed: vacuoles containing food, and vacuoles containing 

 only fluid. The latter form hemispherical protuberances of the surface, 

 and are at all events partially contractile. There is besides a central 

 nucleus, which can only with difficulty be made out in the living ani- 

 mal, in consequence partly of the more or less dark color of the shell. 

 Pseudopodia radiate from all parts of the surtiice through the holes in 

 the latticed shell. They are richly granular threads of extraordinary 



