344 SYNOPTIC COLLECTION. 



that extend forward and lie horizontally, concealing from 

 sight the two arms and the mouth parts. 



Leach says that Crangon vulgaris or the common 

 shrimp often enters estuaries, especially during the breed- 

 ing season, and it sometimes ascends rivers with the tide 

 and is left in great quantities in the saline marshes. 



The lobster, Homarus americanus M. Edw. (PI. 845), 

 passes through the nauplius state in the egg, and when 

 hatched is surrounded by a thin membrane which is 

 moulted before the little creature enters upon a free- 

 swimming life. The first larval stage (PL 845, fig. i) is 

 little over a third of an inch long. It is marked by the 

 presence of a comparatively large cephalothorax, big 

 compound eyes, and a prominent frontal spine ; it also 

 has a segmented abdomen with a fan-like telson. The 

 cephalothorax bears appendages which are two-branched ; 

 the outer branch or paddle is flattened and provided with 

 long hairs, while the inner branch is prehensile. These 

 appendages resemble the swimming organs of the Schiz- 

 opods so closely that this stage is often called the 

 Schizopod larva. The abdomen is without appendages, 

 though their rudiments can be seen under the skin from 

 the second to the fifth segments. In the next larval 

 stage (fig. 2) the cephalothorax and its appendages are 

 similar but the rudiments of abdominal swimmerets have 

 developed. At the fourth moult (fig. 3) the swimming 

 paddles are reduced to vestiges (fig. 4), while the inner 

 branches have developed into walking-legs. The lobster 

 still stays at the surface, swimming forward by its abdom- 

 inal swimmerets and backward by its abdomen. At the 

 sixth moult the thoracic swimming organs are wholly 

 lost and before the seventh moult is passed the lobster 

 leaves the surface of the sea, going to the bottom and 

 approaching the shore where it lives among the rocks. 

 Its larval life is now over. During the first year the lob- 

 ster may moult from fourteen to seventeen times, and at 

 the end of this time it is from two to three inches long. 



