148 COMPARATIVE ANATOMY 



thoracic, and abdominal ganglia can easily be seen ; the 

 first five pairs of post-oral ganglia are still distinct, and have 

 not yet fused to form the composite postcesophageal ganglion 

 of the adult. 



Shortly after this stage, the young crayfish is hatched, with 

 practically the full characters of the adult. There are con- 

 siderable differences, it is true, such as the shape of the 

 rostrum, the dome-shaped carapace, the form of the telson, and 

 the absence of the first and last pair of abdominal limbs, but 

 none of so much importance as to lead us to describe the 

 newly-hatched young as a larval form. As soon as the young 

 are set free from the egg case, they attach themselves to the 

 abdominal limbs of the mother by their chelae, the extremities 

 of which are sharply pointed, and incurved, so that when they 

 once lay hold of any soft substance, the grip cannot easily be 

 relaxed. The limbs of the mother being covered with the 

 tenacious material which formed the egg cases, the young 

 crayfishes are firmly fixed, and are carried about by her until 

 they have completed their first moult. Afterwards they leave 

 their parent from time to time, returning to her protection 

 when alarmed, and eventually, after further ecdyses, they 

 leave her altogether, and lead an independent existence. 



