162 CRUSTACEA—_EUCARIDA—-DECAPODA CHAP. 
in all respects with the Nauplius of the Entomostraca. Secondly, 
the thoracic limbs when they are first developed are biramous, 
thus giving rise to the characteristic Mysis stage which links 
the Macrura on to the “Schizopoda.” Thirdly, the order of differ- 
entiation of the segments is typically from in front backwards, the 
only precociously developed appendage being the sixth abdominal. 
None of these characters are reproduced in the higher Decapoda 
in which there is never a free-living Nauplius, the first larval 
stage being the Zoaea; a number of the thoracic pereiopods, and 
usually all of them, are uniramous from the start ; and the whole 
of the abdominal segments with their limbs tend to be precoci- 
ously developed before the hinder thoracic segments make a dis- 
tinct appearance. 
Tribe 3. Peneidea.' 
The third legs are chelate except in genera in which the legs 
are much reduced. The third maxillipedes are seven-jointed, the 
second maxillipedes have normal end-joints, and the first maxilli- 
pedes are without a lobe on the base of the exopodite. The 
pleura of the first abdominal segment are not overlapped by those 
of the second. The abdomen is without a sharp bend. The 
branchiae are usually not phyllobranchs. 
Fam. 1. Peneidae—The last two pairs of legs are well 
developed, and there is a nearly complete series of gills.  Cera- 
tuspis, a pelagic form.  Parapeneus, Peneus, Aristaeus, ete. 
Fam. 2. Sergestidae——The last or last two pairs of legs are 
reduced or lost. The gill-series is incomplete or wanting. 
Sergestes possesses gills, and the front end of the thorax is not 
evreatly elongated. Lucifer has no gills, and the front of the 
thorax is greatly elongated, giving a very anomalous appearance to 
the animal. All the members of this family are pelagic in habit. 
Fam. 3. Stenopodidae—One or both legs of the third pair 
are longer and much stouter than those of the first two pairs. On 
a number of small anatomical points this family, including the 
littoral genus Stenopus from the Mediterranean and other warmer 
seas and Spongicola commensal with Hexactinellid sponges from 
Japan, is separated by some authors in a Tribe by itself. 
1 Borradaile’s useful paper on the classification of the Decapoda (Ann. Mag. Nat. 
Hist. (7), xix., 1907, p. 457) should be consulted for this and other Decapod 
groups. Also Aleock’s Cat. of the Indian Mus., ‘‘ Decapod Crustacea.” 
* Giard and Bonnier, Compt. Rend. Soc. Biol. 1892. 
