290 CRUSTACEA, 



limbs bud out, wliich is distinguished by the fact that the third pair of 

 inaxillipedes develops simultaneously with the rudiments of the ambulatory 

 limbs (Fig. 135, mf" and p). The pleopoda of the second, third, and fourth 

 abdominal segments are the first to appear, while those of the fifth segment 

 and tlie lateral appendages of the caudal fin only develop at a later period. 



The development of the sand crabs {Hippidae, Fig. 134) also, which was 

 made known by the treatises of F. Muli.er (No. 16), Glaus (No. S), Sidney 

 Smith (No. iri2), and W. Faxon (No. 126), bears a general resemblance to that 

 of the Anomura, although in this case, as in I'orccUana, there are many 

 suggestions of the Brachyuran afiinities. The youngest stage, described by 

 W. Faxon for Hippo, talpoida, is a Zoaca, on whose dorsal shield oidy the 

 rudiments of the later spinous processes can be recognised. The abdomen which 

 is ventrally flexed has the first segment still indistinctly marked oft' from the 

 thorax, and the sixth segment fused with the telson. The latter plate is toothed 

 and rounded posteriorly. The rudiments of the two antennae, the maxillae, 

 and the two anterior maxillipedes used, as swimming limbs, are present, while the 

 third pair of maxillipedes, as well as all the following limbs, are still altogether 

 wanting. From this stage proceeds the Mcta::oaca observed l)y Sidney Smith 

 (Fig. 134), which shows the rudiments of the third pair of maxillipedes (mf") 

 and the four anterior pairs of ambulatory limits {p'-p'^). The fifth pair develops 

 somewhat later. The third pair of maxillipedes is not used as a locomotory 

 organ, although it functions in this way in the larva described by Clatjs and 

 referred by him to Albunea (No. 8). In the Hippn larva, the dorsal spine of the 

 cephalo-thorax, so characteristic of the Brachyuran Zoaeae, is wanting. On the 

 other hand, a long, anteriorly-curved rostrum develops, as well as the lateral 

 posteriorly-directed spines which occur in all Anomura. From the Metazoaea, 

 after several moults, there comes a Mcgalopa very like the adult, being chiefly 

 distinguished from it by the comiiaratively large size of the eyes and the 

 presence of strong biramose swimmerets on the Wve posterior abdominal segments. 



The larvae of the Apterura [Dromia, Homola) also, descrilied by Boas 

 (No. 104) and Goueret (No. 130), are nearly allied to those of the Anomura. 

 Unlike the Brachyuran Zoaca, the third maxillipede in them has an exopodite 

 which functions for swimming ; in Dromia, indeed, tliere is a similar exopodite 

 on the first ambulatory limb, this being a more distinct indication of a Mysis 

 stage than is found in the Anomura. In the shape of the dorsal shield and of 

 the abdomen, which is provided with two ])airs of biramose pleopoda, the 

 Apteruian larvae agree with the Anomura.* 



H. Brachyura. 



Most Brachyura leave the egg in the form of a Zoaea which varies 

 very little in appearance throughout the group (Fig 136). The 

 compact and usually oval body is, as a rule, characterised by the 

 regular development of spines, f There is a frontal spine sloping 

 downwards and forwards, a dorsal spine rising from the centre of 



* [The Anomuran Decapoda were formerly divided up into the Pterygura and 

 tlie Apterura. Of the.se the former (by far tlie larger group) are now classed 

 by Stebbinc. with the Macrura as the Anomalous Macrura, and the latter, 

 comprising the Drominea and Ranininea, as the Anomalous Brachyura. — Ed.] 



t Weldon has pointed out the significance of these spines in checking move- 

 ment in certain directions. Joarn. Mar. Biol. Assoc, N. S., Vol. i. 



