$ 268. THE CRUSTACEA. 313 
the segments of the body is greatest; and most simple when these seg- 
ments are atrophied or blended together. 
§ 268. 
The locomotive organs of the Crustacea are, in general, very numerous ; 
for, often all the segments, from the head to the extremity of the tail, that 
is, the three corresponding to the thoracic segments of insects, and those 
of the posterior part of the body, have, each, a pair of articulated append- 
ages. In the order Myriapoda, the Chilognatha have two pairs of legs on 
each segment of the body.” The form of these organs may be most vari- 
ously modified, and even so much so that their function is entirely changed. 
But those of the first five segments of the abdomen are most constant in 
their form; although they change their function, being sometimes ambula- 
tory legs, sometimes prehensile organs, and sometimes oars. When prehen- 
ile organs, their last joint is armed with a very hooked, sharp claw; when 
oars, this same joint becomes a plate bordered with stiff bristles or bifid hairs. 
The locomotive organs of the three thoracic segments are usually pressed 
towards the mouth and changed into foot-jaws, which serve either as masti- 
catory, or as tactile and prehensile organs. The appendages of the pos- 
terior part of the body may have even yet wider variations. They may be 
changed into false or abdominal feet serving sometimes as oars, as fins, or 
as respiratory organs; and, in the act of generation, they may play the. 
part, some, of copulatory organs, and others, as porters of the eggs. 
When they are ambulatory, or when prehensile organs, these appendages 
may be divided into six pieces, viz.: The Cora, the Trochanter, the Femur, 
the Tibia, the Metatarsus, and the Tarsus the extremity of which, with 
the ambulatory feet, is often prolonged into a short, stiff claw. When they 
serve as natatory organs, the separate joints are more or less flattened and. 
spread out. When used ag prehensile organs, they are either monodactyle 
— the entire tarsus being transformed into a strongly-curved hook which 
can be applied against the metatarsus,—or they are didactyle or like 
pincers, — the metatarsus being thickened or increased in a hand-like man- 
ner, and prolonged into an immovable process (Index), against which the 
tarsus (Pollex) can be applied in a finger-like manner. 
From these metamorphoses and the complete abortion of these append- 
ages, the various forms of Crustacea may be reduced to a few principal 
types, as follows: 
helt 
4 The muscular system is highly developed with 
Decapoda, Stomapoda, Amphipoda, Isopoda, 
Myriapoda, Poecilopoda and Phyllopoda ; see 
Tr ), Rathké, in the Nov. Act. Nat. 
Cur. XIX. p. 141, Tab. XVII. fig. 2. 3 (Dicheles- 
tium) ; Pickering and Dana, in the Isis, 1841, 
Geveke, De Cancri astaci quibusd. partib. p. 7, 
fig. 1-7 ; Suckow, Anat. physiol. Untersuch. loc. 
cit. p. 64, Taf. IX. X. (dstacus fluviatilis) ; 
Milne Edwards, Hist. Nat. d. Crust. I. p. 155, 
Pl. XIL. (Homarus marinus) ; Kutorga, Scolo- 
pendr. morsit. Anat. p. 12, Tab. I. fig. 1, 2; Van 
der Hoeven, Recherch. sur Vhist. nat. et Vanat. d. 
Limulus, p. 24, Pl. IIL; Zaddach, De Apodis 
cancriformis Anat. p. 4, Tab. I. III. 
6 The abortion of the muscular system is often 
80 extensive in the lower parasitic Crustacea, that, 
beside the few muscles belonging to the tactile 
and locomotive organs, there are found only some 
longitudinal and transverse fibres under the skin ; 
see Nordmann, Microgr. Beitrage, Hft. 2, p. 6, 
Waf. I. V. VIL eae at Actheres and 
Taf. IV. (Caligus). 
1 This anomaly, in which the three segments back 
of the head do not participate, is due, perhaps, 
to the segments of the body being always fused in 
twos. 
.2In the interpretation of the movable append- 
ages, I have relied for the most part on the princi-- 
ples of Erichson (Entomograph. loc. cit.), for they 
appear most i and In the. 
instances where, at first sight, they appear un— 
warranted, they may be very well explained by. 
recourse to the phenomena of development of 
Crustacea ; and by this means, here, especially, 
where the metamorphoses occur gradually and 
continuously, may be found the solution of many 
obscure questions in morphology. 
