ASTACIDEA. 275 



already show the rudiments of pincers, and are provided with exopodites, which 

 are wanting on the three following ambulatory limbs. The gills (pleurobranchiae) 

 are well developed on all the segments bearing the ambulatory limbs. The 

 abdominal limbs are found as biramose buds. The telson, in shape and armature, 

 resembles that of the Caridid Zoaea, and is not as yet sharply marked off from 

 the sixth abdominal segment. In contrast to the above, it is an interesting fact 

 that the variety of Palaemonetes varians, which occurs on the shores of Northern 

 Europe (but also in brackish or fresh water), shows far less abbreviation of 

 metamorphosis. According to Boas (No. 105), the Zoaeae which hatch from 

 the eggs of this form have only truncated, unjointed ambulatory limbs, while 

 the abdominal limbs are altogether wanting. In all forms with abbreviated 

 development, the eggs are considerably larger and more richly provided with 

 food-yolk, and the number produced by the female is consequently smaller. 

 The young, when hatched, still retain a considerable amount of food-yolk, 

 which is only gradually absorbed, and consequently they do not take food from 

 outside until very late. In keeping with this, the mouth -parts of the otherwise 

 highly-developed larva are in a very rudimentary condition. 



According to Herrick (No. 133) and Packard (No. 144) a similar abbrevia- 

 tion of metamorphosis is found in two species of Alplieus. In other species of 

 this genus the metamorphosis does not deviate from that of other Caridea, 

 but in Alpheus hcterochelis it is abbreviated, and in Alplieus praecox, which lives 

 in Sponges, it is almost altogether lost. 



In connection with the Caridea we should mention the genus Amphion, a 

 form whose systematic position is still very doubtful. It agrees in the form of 

 its Zoaea larva and in the possession of branchial rudiments which show the 

 character of phyllobranchiae (Spence Bate, No. 100) with the Caridea. The 

 oldest known stages of Amphion appear (in the deficient segmentation of 

 the antennae) not to have attained full development, and must probably be 

 >nsiclered as larvae, although Dohrn (No. 120) and Willemoes-Suhm, on 

 account of the statement that genital rudiments appear in them, were inclined to 

 regard them as adult forms. The long body moves by means of six pairs of 

 biramose limbs, which correspond to the second and third maxillipedes and the 

 four following thoracic limbs, while the last pair of thoracic limbs is present 

 in an undeveloped condition. In the shape of its biramose limbs, as well as in 

 the presence of branched hepatic tubes in the anterior part of the cephalo-thorax, 

 AmjtMon shows striking agreement with the Phyllosoma, and Boas (No. 103) 

 actually conjectured that it might be the larva of the genus Polycheles, which is 

 related to the Loricata. The youngest known stages of Amphion are Zoaeae 

 resembling in appearance the Caridid Zoaeae. The second and third pairs of 

 maxillipedes, developed as biramose swimming limbs, are the principal loco- 

 motory organs, while the first pair of maxillipedes already appear drawn 

 towards the mouth. The abdomen ends in an oval telson ; the pleopoda are 

 wanting, but the sixth pair very soon develops. Other forms nearly related to 

 Amphion have been described by Willemoes-Suhm, their similarity in shape 

 of body to Scrgcstes being pointed out. These forms are known as Amphiones. 



D. Astacidea. 



In the tribe of the Astacidea there is still greater abbreviation of 

 metamorphosis than in most Caridea. There is no longer any free 

 Zoaea stage. In Homarus, whose metamorphosis has become known 



