PODOPHTHALMIA. 49 



and abdominal feet are usually absent. These zoeas differ from those of the Brachyura 

 in that the enormous defensive spines, so characteristic of the larvae of that sub-order, 

 are here but very slightly developed, thus enabling one at the first glance to say to 

 which group any larva belongs. From the zoea a Mysis-Y\ke. stage is produced, in 

 which the thoracic and finally the abdominal feet appear, the thoracic feet exhibiting 

 as a temporary character the schizopodal form, which is retained in the adult opossum- 

 shrimp. From this the change to the adult condition is gradual. To this one ex- 

 ception may be noted ; in the hermit crabs (Paguridea) and Thalassinidea, so far as 

 known, the Mysis stage has disappeared. 



For our purposes we may divide the Macrura into the groups Penasidea, Caridea, 

 Astacidea, Tlialassinidae, Galathseidea, and Paguridea, the two latter embracing a por- 

 tion of the old group Anomura, each in turn being divisible into several families. 



The PEN^IDEA, with its two families Pen^id^ and Sehgbstidab, though a 

 well-marked group, is not easily defined in words, if we rely upon characters derived 

 wholly from the adult, but the three genera, whose development has been studied, 

 Lucifer, Sergestes, and Penceus, all leave the egg in the nauphus condition, and are thus 

 in strong conti-ast to all the other Decapods. Aside from their development, — a slight 

 account of which will be given, — the whole group possesses but few points of interest. 

 Of Lucifer, a small transparent form, one species of which is found oli our southern 

 coast, we have, thanks to that able naturalist. Dr. Brooks, one of the most complete 



life-histories yet published. The eggs, contrary to 



"^^^^^>'«>>,^^ ^^__^,^^ ^^'^''^^^'^ those of most Crustacea, are almost entirely com- 



^°**=-^a,^^ y^^ <.^<*°°*^ posed of protoplasm, and undergo a total segmenta- 



/: V J ~"V^ ^\ar\, followed by the formation of a true segmentation 



/\i^ ^^s*' r^^L \ cavity. From the egg there hatches a true nauplius, 



[Jtc Ml r~€^ N akJ which, by two moults, produces a protozoea with 



U vS "v^ I V^ / n ^° elongate but unsegmented abdomen, and a well- 



Vv \_x^vV // developed carapax. Seven pairs of appendages are 



now present, while the segments of the body cor- 



FIG. 68. — Nauplius of Z/Mci/er. ^^ ' , . -, .„. , , ,, ,, 



respondmg to the third maxilliped and the three 

 first thoracic limbs are outlined. The heart is formed, and Dr. Brooks thinks that 

 water was drawn in and expelled from the posterior portion of the intestine, a feature 

 which would remind us of the intestinal respiration already mentioned in connection 

 with Mysis on a preceding page. "With three moults, during which nearly all of the 

 thoracic and abdominal segments appear, while the compound eyes are developed (in 

 a manner which, if we rightly interpret the text and drawings of Dr. Brooks, lends not 

 the slightest countenance to the idea that they are homologically jointed appendages), 

 the zoea istreached. From this point the development is much more gradual, the 

 larva passing through a Mysis stage, and reaching essentially the adult form when 

 about half an inch long. 



The development of Penceus, so far as known, corresponds in a general way with 

 that of Lucifer, while in Sergestes some of the larval stages are characterized by very 

 peculiar branching spines. 



The species of Penceus, all of which have the three anterior pairs of feet chelate, 

 are very numerous in the warmer seas of the globe, and form an important article 

 of food. In the Southern States large numbers of Penceus braziliensis are sold under 

 the name of shrimp. 



The only other form of Penseoid which needs mention is the curious Spongicola 



VOL. II. —4 



