314 THE CRUSTACEA. $ 268. 
1. With the Myriapoda, these appendages are ambulatory and have the 
same form with all the segments of the body ; and only with the Chilopoda 
the anterior and middle pairs of the first segment corresponding to a 
thorax, are changed into tactile organs, 
2. With the Isopoda, Laemodipoda, and Amphipoda, the first thoracic 
pair are tactile organs. With the Amphipoda, the second and third 
thoracic pairs are changed into prehensile organs armed with a claw. The 
five anterior abdominal pairs are ambulatory and unchanged, with the Iso- 
poda, and Amphipoda. But the remaining posterior pairs are transformed, 
with the first of these orders, into lamelliform respiratory organs ; and with 
the second, into short, very movable appendages, terminated, each, by a 
double uni- or multi-articulate cirrus, which serve sometimes as oars, some-- 
times as gyratory organs. 
3. With the Decapoda, the thorax is entirely abortive, and its three. 
pairs are changed into oral and tactile organs; while the first pair, belong- 
ing to the anterior abdominal segments, is usually transformed into a for- 
ficulate prehensile organ. The four succeeding pairs are simply ambulatory 
organs. But the appendages of the posterior part of the abdomen are re- 
duced to tendril-like processes, which play a part in the act of generation. 
With the Squillina, the three thoracic, and the first two abdominal pairs 
have the form of prehensile organs, while the three succeeding pairs retain 
their character of ambulatory organs, and those of the remaining posterior 
segments are changed into lamelliform fins. 
4. In the section of the inferior Crustacea, designated usually under the 
name of Entomostraca, the head and thorax are fused into a single part 
ealled Cephalothorax, and the mouth is situated so far behind, that the first’ 
pair of feet is in front of it. The locomotive apparatus here consists 
usually of oars or prehensile organs. With the Poecilopoda, the first 
three pairs of appendages are forficulate, as, also, are the three pairs of 
jaws. With the Phyllopoda, and Lophyropoda, the first two pairs of feet 
resemble antennae; of these sometimes the first, as well also as the second, 
which are usually branched, serve as oars; ® here, also, the often very 
numerous, anterior abdominal appendages are used usually as fins, while the 
posterior ones are scarcely at alldeveloped. , 
5. With the Cirripedia, the first thoracic pair is transformed in a 
remarkable manner. With the Lepadea, they are changed into a soft 
foot; and with the Balanodea, into a shell. The remaining six pairs 
are multiarticulate cirrate organs, and the abdomen is prolonged into a tail 
free from appendages. The three anterior pairs of these cirrate organs are 
the shorter, and have a tactile function; while the three posterior are used 
as gyratory organs. 
6. With the Siphonostoma, the mouth is even still further behind, and 
8 With Cyclops, Cyclopsina, and Cypris, it is 
the first pair of feet that is transformed into oar- 
like organs, but it is the second with Apus, Lim- 
nadia, Daphnia, and Polyphemus. 
With the Branchiopoda, the disposition is quite 
different. The anterior pair is changed into two 
slightly movable appendages, hook-like or digiti- 
form, and rolled spirally. With the embryos and 
* ( § 268, note 3.) The first pair of feet is, gene- 
rally, the second pair of antennae. For a full dis- 
the young animals, these organs are used clearly as 
oars; see Jurine, Hist. d. Monocles, loc. cit. Pl. 
XX. fig. 9, and Pl. XXI. fig. 1, 2(Chirocephalus), 
and Joly, in the Ann. d. Sc. Nat. XIII. Pl. VI. 
(Artemia).* 
4 See Thompson, Zool. Research. Pl. IX. fig. 3 
(Balanus), and Burmeister, Beitr., &c., Taf. I. 
fig. 3-5 (Lepas). 
cussion of this point, see Dana, Report on Crustac.. 
&e. p. 1031.— Ep. 
