290 CRUSTACEA. 



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

 maxillipedes 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 the 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. Muller (No. 16), Claus (No. 8), Sidney 

 Smith (No. 152), and W. Faxon (No. 126), bears a general resemblance to that 

 of the Anomura, although in this case, as in Porcellana, there are many 

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

 W. Faxon for Hippa talpoida, is a Zoaea, on whose dorsal shield only the 

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

 is ventrally flexed has the first segment still indistinctly marked off 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 Metazoaea observed by Sidney Smith 

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

 and the four anterior pairs of ambulatory limbs {}) r -p iy ). 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 Claus and 

 referred by him to Albunea (No. 8). In the Hippa 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 Megalopa very like the adult, being chiefly 

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

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



The larvae of the Apterura (Dromia, Homola) also, described by Boas 

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

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

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

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

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

 the abdomen, which is provided with two pairs of biramose pleopoda, the 

 Apteruran 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 

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

 by Btebbing with the Macrura as the Anomalous Macrura, and the latter, 

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



I Wki.hon has pointed out the significance of these spines in checking move- 

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



