APPENDIX. 305 



a line in diameter, and of a dingy yellow colour, and with a mark on 

 each side. 



PISCICOLA (page 42). 



M. de Blainville says of this genus that there are only four ocular 

 points in two pairs ; Moquin-Tandon says eight, "reunis par paires 

 et disposes en trapeze," — the peculiarities, perhaps, of distinct spe- 

 cies, and not generical. It is, indeed, evident that the character 

 assigned to the genus is derived from one species onl}'^ ; and it ought 

 to be made more general. Tandon says the body consists of 63 

 rings, which is again clearly a specific character. Blainville finds in 

 a species a median band of very minute horny hooks on the ventral 

 s\irface, and he makes this character generical ; and Moquin-Tandon 

 introduces it with a query as to the correctness of the observation. 

 • M. Gervais asserts that the ocular points or specks on the anterior 

 extremity and on the tail of this species are precisely identical in 

 structure! — jlnn. des Sc. nat. vii. 56 (1837). 



Piscicola percse (page 43). 



" It differs from the P. geometra of Linnseus in many particulars ; 

 among others, in having the disk with 14 rays and dark points." — 

 Tem'pleton. 



Nephelis octoculata (page 44). 



The young are greyish, pellucid, with dusky interranea, forming 

 an ill-defined fascia along each side. The sides are then very mi- 

 nutely and neatly crenulated. 



This leech is active. I have rarely found it at rest*. It moves 

 exactly in the same manner as the horse-leech, and, like it, swims in 

 the water by an undulatory or serpentine motion of the body. It is 

 found on and under stones, and often on plants. Miiller says that 

 it swallows microscopical animalcules ; and he has frequently seen it 

 devour the shelled moUusca that are the common inhabitants of the 

 same waters. Johnson tells us that it feeds on small " earth-worms," 

 which it swallows whole. We presume Dr. Johnson intends the 

 small worms which used to be referred to the genus Lumbricus^ and 

 which are strictly aquatic. The structure of the mouth apparently 

 unfits our leech for eating solid food, or biting through a skin ; but 

 Sir John Dalyell says, " it is a fierce, active, and voracious creature j 

 feeding greedily on flesh, and even waging a destructive warfare 

 against its own tribe." " In the natural state it seems to subsist 

 also on the succulent plants, and on decaying wood, but always pre- 

 ferring animal substances." 



The N. octoculata is oviparous ; and it affixes the capsule, which 



* Duges tells us that it will often remain for hours, or even entire days, fixed 

 by its posterior sucker, waving and keeping the body, slightly flattened, in a con- 

 stant undulation, — apparently to renew the water in contact with the surface, and 

 aid respiration. 



X 



