158 W. H. HARRIS ON MARINE MICROSCOPIC VEGETABLE ORGANISMS. 



others which are almost straight and apparently more deeply immersed 

 within the shell containing the organism, but further investigation is 

 necessary. 



The filaments appear to be devoid of appendages. 



Widely distributed, but not very common ; not observed below 45 fathoms. 



Aclilya miniitula. — Exceedingly minute plants, usually about one-fiftieth 

 of an inch in diameter when mature, occurring generally in groups much 

 crowded together. 



The plant branches freely, and the filaments have an undulating appear- 

 ance, the true cause of which I have been unable to determine ; no 

 appendages are visible by any power I have been able to use. 



Widely distributed. Rare within the British area. 



Not observed below 120 fathoms. 



Achlya gracilis. — The distinguishing characters of this species appear to 

 be more uniformly persistent than in many of the members of the group. 



The filaments may be divided into three distinct groups, which always 

 occupy separate portions of the shell they invade. 



The contour of the plant is displayed at or near the surface of the nacreous 

 layers ; at frequent intervals branches are given off which descend vertically 

 into the shell ; in some instances these terminate in irregular-shaped cavities 

 (to which further reference will be made), while others pass on until they 

 sometimes reach the opposite surface, where they may terminate, or they 

 may be deflected and return for some distance towards the original point of 

 entry. 



These filaments may be simple and hair-like, or they may be slightly 

 branched. 



The superficial aspect of the plant conveys the idea of rigidity, caused by 

 the uniformity with which the branches divide. 



The filaments are exceedingly small in diameter, and at the point where 

 they descend into the shell they appear to be jointed, but I am not satisfied 

 on this point. 



The vesicles which terminate the short branches are generally contained 

 wholly within the nacreous portion of the shell ; in their perfect condition 

 they are spherical, and are provided with an excretory duct, but they appear 

 to speedily erode the shell and form irregular-shaped cavities ; with the 

 exception of the excretory duct referred to, the plant appears to be destitute 

 of appendages for communicating with the surrounding water; in this 

 respect it appears to occupy an intermediate position between the group so 

 provided and that which is quite destitute. 



Widely distributed, but rare outside the British area. Plate 9, Fig. 7 

 X 300. The figure exaggerates the diameter of the filaments. 



Aclilya fluxuosa. — This organism possesses nearly all the features of 

 A. gracilis, but instead of having the rigid appearance of the latter, the 

 superficial branches are beautifully curved ; the individual plants are smaller 

 and nearly circular in contour ; it appears to quickly erode the surface of 

 the shell, leaving continuous channels which present a serrated appear- 

 ance at the margins. 



