464 
7 RANSFORMA TIONS OF INSE CTS. 
were carefully pointed out by the same accurate observer, and 
then he proved that Praniza had eggs 'of different shape to the 
last-mentioned genus. Praniza , to have eggs, must be an adult 
form, and therefore could not be the immature zoea of Anceus. 
Whilst accepting Spence Bate’s proofs of the distinctness of the 
two* forms, it must be remarked that M. Hesse had some warrant 
for his idea that the supposed change from the Praniza stage to 
that of Anceus took place very late in the life of the creature, 
for a correspondingly great alteration does occur in other Iso- 
poda, such as Tanais, the males of which receive a very con- 
siderable access of abdominal feet simultaneously with the last 
pair of feet of the thorax, and alter greatly before attaining 
sexual maturity. 
There are some examples of great retrograde metamorphosis 
amongst this order of the Crustacea , as a consequence of a parasitic 
mode of life. The young of the Fish Lice ( Cymothoa ) are lively 
swimmers, and the adults stiff, stupid, heavy fellows, whose short, 
clinging feet are capable of but little movement. Some, which are 
parasitic upon crabs and lobsters, take up their abode chiefly in the 
branchial cavity, and then it is found that the adult females are 
usually quite destitute of eyes. The antennae are rudimentary, 
the segments of the body are more or less amalgamated with each 
other, the feet are stunted, and the swimming appendages are 
transformed into leaf-shaped branchiae. In the males, which are 
dwarfed, the eyes, antennae, and feet are usually better preserved 
than in the females ; but, on the other hand, all the appendages 
and every trace of segmentation have not unfrequently disappeared. 
In the females of Entoniscus , which are found in the body cavity of 
crabs, the eyes, antennae, mouth-organs, the segmentation, and the 
whole of the limbs (in one species) disappear without leaving a 
trace. One species would almost be regarded as a flat worm 
rather than an Isopod ’ if its eggs and young did not betray its 
crustacean nature.* 
Metamorphosis appears to be absent in the common wood- 
louse, which may be taken as the type of the Isopoda. 
The small water fleas, or Daphniidce , are well known to those 
Fritz Muller, translated by W. S. Dallas. 
