CRUSTACEA. 395 



Amongst the other Macrura the larva generally leaves the egg as 

 a Zosea similar to that of the prawns. In the case of the Thalassinidai 

 and Paguridse a Mysis stage has disappeared. The most remarkable 

 abbreviations of the typical development are presented on the one 

 hand by Homarus and Astacus, and on the other by the Loricata. 



The development of Homarus has been fully worked out by S. J. Smith 

 (No. 491) for the American lobster (Homarus americanus). The larva (fig. 

 221) leaves the egg in an advanced Mysis stage. The cephalo-thoracic shield 

 is fully developed, and armed with a rostrum in front. The first pair of 

 antennae is unjointed but the second is biramous, the outer ramus forming 

 a large Mysis-like scale. The mandibles, which are palped, the maxillae, 

 and the two anterior maxillipeds differ only in minor details from the same 

 appendages of the adult. The third pair of maxillipeds is Mysis-like and 

 biramous, and the five ambulatory legs closely resemble them, the endopo- 



FIG. 221. NEWLY-HATCHED LARVA OF THE AMERICAN LOBSTER. (After Smith.) 



dite of the first being imperfectly chelate. The abdomen is well developed 

 but without appendages. The second, third, fourth and fifth segments are 

 armed with dorsal and lateral spines. 



In the next stage swimming feet have appeared on the second, third, 

 fourth and fifth abdominal segments, and the appendages already present 

 have approached their adult form. Still later, when the larva is about 

 half an inch in length, the approach to the adult form is more marked, 

 and the exopodites of the ambulatory legs though present are relatively 

 much reduced in size. The svvimmerets of the sixth abdominal segment 

 are formed. In the next stage observed the larva has entirely lost its 

 Schizopod characters, and though still retaining its free swimming habits 

 differs from the adult form only in generic characters. 



As has been already stated, no free larval stages occur in the deve- 

 lopment of Astacus, but the young is hatched in a form in which it differs 

 only in unimportant details from the adult. 



