20 



LECTURE II. 



Navicula. 



Dictyocha. 



Actinocyclus, bivalve. 



their flinty armour resem- 

 bling a minute bivalve shell: 

 in some, as the Navicula, it 

 has the form of an elongated 

 case, or flattened cylinder, 

 open at both extremities {fig. 8.) : it is sometimes straight, some- 

 times bent, like the Australian boomerang ; it may present the form 

 of a reticulated cone {fig. 9.), or a discoid case 

 {fig. 10.) : in short, the 

 varieties of the siliceous 

 shells of the Infusoria 

 are as numerous as those 

 of the calcareous shells 

 of the Mollusca. But 

 whatever their form, the 

 superficies of these delicate microscopic objects is generally sculptured 

 with a beautiful, well defined, and more or less complicated pattern, 

 which makes it easy to recognise the species, and distinguish them 

 from one another. 



Most of these animated minims are locomotive and free ; a few^, as 

 the VorticellcB, are attached to foreign bodies by a long and highly 

 irritable and contractile pedicle ; others, as the Gomphonema, are 

 appended to the extremities of the branches of a dichotomously di- 

 vided stem. 



In Loxodes Bursaria (fig. 18.) the parietes of the body, which 

 equal about one-sixth of its transverse diameter, consist of an outer, 

 firm, colourless substance supporting the cilia, and of an inner, softer 

 matter, in which green globules are imbedded. The outer layer is 

 marked by numerous close-set, fine grooves, extending obliquely or 

 spirally, and in opposite directions, over the whole outer surface, 

 giving it an " engine-turned" or reticulate character. The cilia are 

 attached to the eminences defined by the decussating grooves, and are 

 of considerable length, when seen in the quiescent, dying, or decom- 

 posing animacule.* 



The green corpuscles imbedded in the deeper layer are spherical 

 nucleated cells, corresponding in their chemical characters with the 

 chlorophyll-corpuscles of Vancheria and other Algae ; the similar cor- 

 puscles which float in the fluid contents of the body are seen in cir- 

 culation close to the inner surface of the parietes, in the direction 

 indicated by the arrows in fig. 1 8. 



The locomotive Polygastria propel themselves through the water 

 by the action of their vibratile cilia, Avhich are sometimes generally 



* XVIII. t. xxxiv. fig. vii. Bursaria venialis. 



