CRUSTACEA. 
281 
find demonstrated by Vaughan Thompson to be the larval con- 
dition of the common crab of our coast, many important and 
interesting observations on the morphological development of 
Crustacea have been made by different naturalists ; but vre think 
that few have been more unanticipated than the recent dis- 
covery of Dr. Fritz Muller on Penaeus. 
We have long since been enabled to distinguish upon general 
principles between the larvie of Erachyurous, Anomurous, and 
Macrurous Decapods ; but from the characteristic forms of the 
larvfc of these orders some very remarkable departures have 
been discovered by naturalists. 
The first, which are as important as any, are the researches of 
Eathke on the Crayfish {Astacufi), and Professor Westwood 
on the Land-Crabs of the West Indies [GecMrcinus) , in which 
we find the apparently tiniversal law of the development of 
Crustacea set aside, in order that the young of freshwater 
decapods may assume a general resemblance to the parent 
before it is entrusted with the care of its own existence. 
For some time observation remained at a standstill, as if 
naturalists had arrived at the limit, and exhausted research 
upon the subject. But at the Meeting of the British Association 
held at Dublin (1857), the late Mr. R. Q. Couch announced the 
curious and important discovery that the larvse of Palinurus vul- 
garu bore a very close general resemblance to the adult form of 
tlie genus Phylhsoma. Here we see another remarkable and 
extreme variation in the morphology in this order of Crustacea ; 
and still more recently Dr, Fritz Muller tells us that the larva 
of the genus Penams, if not that of other genera of the Prawns, 
passes through a morphological development, of which the earliest 
form assimilates very closely in its general appearance to that 
of the larval forms of the Cirripedia, Lerncans, and some ento- 
mostracous Crustacea, before it arrives at that which we are 
inclined to regard as the normal type of the Macrurous larva. 
Great numbers of the larvie of Crustacea were observed by 
him to be swimming about in the sea during tlie southern summer 
months ; some of these he was enabled to trace from the youngest 
stage to that of the adult form. It is true that in the chain 
there are one or two links wanting to make the connexion perfect, 
which at no distant period we hope that he will be fortunate 
enough to add*. 
In the youngest stage the larva is destitute of distinct somites ; 
it is pyriform, 0 4 mill, in length, rounded in front, and 0 2 
mill, in breadth at the widest part, wJiich is just behind the head, 
and gradually narrows posteriorly until at its narrowest, where 
the breadth is just j^th of the length of the animal — tliat is at the 
* Since this has been in type, Pr. Miiller informs ns that the soAeral links 
in the progressive development have been established by him, closer than, 
for want of space, he has been able to demonstrate in his work. 
