§ 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 Cbilognatha 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- 

 sile 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 Coxa, the Trochaiiter , 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 as 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 : 



4 The muscular system is highly developed with Tracheliastes) , Rathki, in the Nov. Act. Nat. 



Decapoda, Stomapoda, Amphipoila, Isopoda, Cur. XIX. p. 141, Tab. XVII. fig. 2. 3 {Dicheles- 



Myriapoda, Poecilopoda and Phyllopoda ; see tium) ; Pickering and Dana, in the Isis, 1841, 



Geveke, De Cancri astaci quibusd. paitib. p. 7, Taf. IV. {Caligus). 



fig. 1-7 i Siickov!, kwal. physiol. Unteisuch. loc. 1 This anomaly, in which the three segn>ents back 



cit. p. 04, Taf. IX. X. (Astacus Jluviatilis) ; of the head do not participate, is due, perhaps, 



Milne Edwards, Hist. Nat. d. Crust. I. p. 155, to the segments of the body being always fused in 



PI. XIII. {Homarus marinvs) ; Kvtorga, Scolo- twos. 



pendr. morsil. Anat. p. 12, Tab. II. fig. 1, 2 ; Van 'i In the interpretation of the movable appcnd- 



ier Iloeven, Recherch. sur I'hist. nat. et I'anat. d. ages, I have relied for the most part on the jjrinci- 



Limulus, p. 24, PI. III.; Zaddach, De Apodis pies of £r!cAson (Entomograph. loc. cit.), for they 



cancriformis Auat. ]). 4, Tab. I. III. appear most consistent and unconstrained. In the 



£ The abortion of the muscular system is often instances where, at first sight, they ajiptar un- 



so extensive in the lower parasitic Crustacea, that, warranted, they may be very well explained by 



beside the few muscles belonging to the tactile recourse to the phenomena of development of 



and locomotive organs, lliere are found only some Crustacea; and by this means, here, cs))ecially, 



longitudinal and transverse fibres under the skin ; where the metamorphoses occur gradually and 



Bee Nordmann, Wicrogr. Ueitrage, lift. 2, p. 6, continuously, may be found the SQlution of many 



Taf. I. V. VII. {Lamproglena, Adheres and obscure questions in morphology. 



27 



