AND SOME VARIETIES OF MELICERTA RINGEN3. 125 



sents itself ; and if I shall not be considered too much infected 

 with Darwinian doctrines, I would hazard the assertion that these 

 three species, together with Limnias, have but one common origin. 

 Traced downwards from Melicerta ringens, through its varieties 

 (drawings of some of which are laid on the table for comparison 

 with the connecting link which the subject of our notice supplied), 

 it will be seen that by easy stages of degradation, through arrest 

 of development or suppression of parts, Limnias ceratophylli will 

 at length be arrived at. 



In the variety of Melicerta, No. 1 (Fig. 2, pi. 1 ), we have an 

 animal, though otherwise perfect, in which the cup-like rotary 

 organ, which, for the sake of expressive phraseology, we will call 

 the " Pug-mill," is wholly wanting. It results, therefore, that 

 unable to mould pellets and construct the neat wall of masonry of 

 Melicerta jsroper, it is invested only with the ordinary amber coloured 

 gelatinous secretion. 



In variety No. 2 (Figs. 3 and 4, pi. 7), a further arrest of de- 

 velopment has taken place ; the four-lobed rotary disk of Melicerta 

 is, though still ample, reduced to two, and without " Pug-mill," 

 only rudely shaped excrementitious masses adherent to the gelatinous 

 investment are observed. 



In the third variety (Fig. 1,2^1- 7), these conditions of degrada- 

 tion are still more advanced — no " Pug-mill," an undivided ciliary 

 organ, and but a single siphon. 



Passing by Limnioides myriophylli (Figs. 3, 4, and 5, pi. QJ, 

 which we have already endeavoured to connect with Melicerta, we 

 come to Cephalosiplwn Limnias (Figs. 6 and 7, pi. 6J, with two 

 smaller rotary lobes and single siphon, which may possibly be formed 

 by the coalescence or soldering together of two siphonal tubes into 

 one long conspicuous canal, with a much contorted viscid sheath, 

 coated with extraneous substances ; and, lastly, we descend to 

 Limnias ceratopliylli (Figs. 1 and 2, pi. 6 J, with two small lobes 

 and water vascular canals wholly suppressed. 



DESCRIPTION OF PLATES. 

 > *»♦- < 



Plate 6, Fig. 1 & 2, Limnias 



ceratophylli X 100 



Figs. 3, 4, 5, Limnioides 



myriophylli X 190 



Figs. 6 & 7, Cephalosi- 

 phon Limnias X 100 



Plate 7, Fig. 2, Melicerta 



ringens... Var. No. 1 X 190 

 Figs. 3, 4... Var. No. 2 X 100 

 Fig. 1 ...Var. No. 3 X 100 



