CRUSTACEA. 4/7 



Thalassinidae and Paguridae 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- 

 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 swimmerets 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 develop- 

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

 in unimportant details from the adult. 



The peculiar larval form of the Loricata (Scyllarus, Palinurus) has long 

 been known under the name Phyllosoma (fig. 222 C), but its true nature was 

 first shewn by Couch (No. 474) [Couch did not however recognise the 

 identity of his larva with Phyllosoma ; this was first done by Gerstacker] 

 and shortly afterwards by Gerbe and Coste. These observations were 

 however for a long time not generally accepted, till Dohrn (No. 477) 

 published his valuable memoir giving an account of how he succeeded in 

 actually rearing Phyllosoma from the eggs of Scyllarus and Palinurus, and 

 shewing that some of the most remarkable features of the metamorphosis of 

 the Loricata occur before the larva is hatched. 



The embryo of Scyllarus in the egg first of all passes through the usual 

 Nauplius stage, and then after the formation of a cuticle develops an 

 elongated thoracico-abdominal region bent completely over the anterior 

 part of the body. There appear moreover a number of appendages and the 

 rudiments of various organs ; and the embryo passes into a form which may 

 be described as the embryonic Phyllosoma stage. In this stage there are 

 present on the anterior part of the body, in front of the ventral flexure, two 



