XII] ASTACUS 201 



at the same time becomes lighter in colour. Such muscles are 

 able to carry out much more rapid contractions than is the case 

 with the unstriped smooth muscles, and as they recover from the 

 state of contraction with great rapidity many quickly succeeding 

 contractions can be carried out. In the case of the Mosquito, 

 which produces its note by the vibration of the wings, it has been 

 calculated that at least 5000 contractions a second must be 

 effected by the wing-muscles. Speaking broadly, muscles of this 

 type are found only in Arthropoda and Vertebrata, but they occur 

 in other groups of the animal kingdom in the case of isolated muscles 

 (cf. jaw-muscles of Echinoidea), where rapid and powerful movements 

 are required. 



The muscles are inserted m the skin, but in the thorax this is 

 folded inwards between the segments so as to produce wedges 

 termed apodemes which project into the body cavity and give 

 attachment to the flexor muscles. The apodemes are, of course, 

 stiffened by a deposit of chitin between the two layers of ectoderm 

 of which they are composed. Between all the segments which bear 

 the "legs," except the last, two apodemes project inwards on each 

 side, one arising from the side, called the endopleurite, and one 

 from the ventral surface, called the endosternite. The two endo- 

 sternites of opposite sides meet in an arch, enclosing the sternal 

 canal, in which lies the nerve cord. On its outer side the endo- 

 sternite forks and one fork, termed the arthrodial apodeme 

 (ap.arth, Fig. 71), meets a corresponding process of the endopleurite, 

 and the two form a strengthening wall round the insertion of the 

 limb. The endosternite has a third process, termed the para- 

 phragmal (ap.par, Fig. 71), which passes forward to unite with 

 the endopleurite in front, so that a somewhat complicated internal 

 skeleton results. At each ecdysis the chitin which stiffens the 

 apodemes is moulted, and at that time the apodemal fold could 

 be straightened out. In some Crustacea, though not in the cray- 

 fish, very marked changes in the shape of the animal are brought 

 about at ecdysis by the smoothing out of apodemes, and the 

 development appears to proceed by sudden jerks. 



Connective tissue has already been mentioned in the case of the 

 Platyhelminthes, Rotifera and Annelida, but in these animals it attains 

 little development compared with that which it reaches in the case 

 of the Arthropoda. It consists, it will be remembered, of a semi- 

 fluid ground-substance, corresponding to the jelly of Coelenterata, 

 which becomes invaded by amoebocytes. These by their metabolism 



