204 DESCRIPTION OF [Polygastrica. 



Whether the two openings on the ventral surface are 

 mouths, and the two on the back apertures for respiration, 

 is undecided ; but the opening on the back, opposite the 

 central ventral opening, is supposed by Ehrenberg a 

 sexual one. No direct demonstration of the nutritive 

 apparatus has yet been effected by using coloured food, 

 though numerous scattered and colourless vesicles are to 

 be seen within the bodies of several species, which indi- 

 cate polygastric structure ; but what Corda took for an 

 alimentary canal (in Pharyngo glossa) was merely the 

 dark central longitudinal furrow of the lorica. This genus 

 is more complex in its structure than the two preceding ; 

 even the majority of botanists consider these beings as 

 animals. The green, yellow, and brown colouring matter 

 in their interior are supposed to be ova. It is in the 

 form of broad plates or fillets, from two to four (8 ?) 

 jointed together in the middle, occupy the interior of each 

 lorica. These plates take the exact form of the interior 

 of the shell, filling the cavities of the flutings, furrows, or 

 striae. In many species, two or four round vesicles are 

 seen, which, although they are not changeable in form, or 

 contractile, yet are sometimes present and sometimes 

 absent, and are probably analogous to small seminal 

 glands. Many Navicula multiply by spontaneous self- 

 division, in which case it is invariably longitudinal and 

 dorsal, or lateral; the division taking place beneath the 

 hard epidermis, as in Gallionella and Achnanthes, and the 

 lorica separating afterwards. It is seldom in this genus 

 that a second self-division commences before the first is 

 complete and separates ; indeed, species, whose indivi- 

 duals separate into four, should be placed in Fragilaria. 



Fourteen of the living forms of Navicula have been 



