Mr Grant 07i the Ova ofihePontobdella muricata. 161 



]atinous matter within is at first thick and of a milky colour, 

 but becomes more colourless, transparent, and thinner, as the 

 Pontobdella approaches maturity, as I have observed in the 

 ova of many molluscous animals. The young animal is visi- 

 ble when only about half a line in length, in the middle of the 

 gelatinous matter, and only one young Pontobdella is found in 

 each ovum. The animal is at first quite transparent and of 

 a whitish colour ; and when ready to escape, it is nearly an 

 inch in length and has all its external characters distinctly 

 marked. We find these ova generally in groups of thirty or 

 forty adhering to solid bodies in deep water where the Pontob- 

 della resides. The merit of having first ascertained them to 

 belong to that animal is due to my zealous young friend Mr 

 Charles Darwin of Shrewsbury, who kindly presented me with 

 specimens of the ova exhibiting the animal in difierent stages of 

 maturity. They nearly resemble the ova of the Amphitrite de- 

 scribed and figured by Basterus, {Op. sub. PI. V. Fig. 1. A, 

 B, C, and p. 38.) The ova of the Pontobdella have pro- 

 bably escaped notice from the animal generally frequenting 

 the bottom of deep water, where it lives by sucking the blood 

 from the surface of flat fishes ; or they may have been mis- 

 taken for young marine plants, such as the Fucus loreus, 

 from the fibrous appearance and deep-green colour of their 

 outer capsule. The description of the ova of the lower ani- 

 mals forms an interesting, though much neglected part of 

 the history of the species ; and an imperfect acquaintance with 

 this part of zoology has sometimes led distinguished natural- 

 ists to mistake the ova of marine animals for zoophytes, and 

 zoophytes for the ova of animals. 



Plate II. Fig. 6, Entire ovum of the Pontobdella muricata, 

 of the natural size, showing the two round lateral promi- 

 nences, the narrow fibrous stem, and thin spreading base. 



Fig. 7, Ovum of the P. muricata divided horizontally, to 

 show its double covering, and the young animal coiled up in 

 the gelatinous matter within. 



VOL. VII. NO. I. JULY 1827. 



