TYPE CRUSTACEA. 375 



the biramous that it is most convenient to regard the latter as 

 the typical condition in the Crustacea. 



Respiratory organs are not always present, but when they 

 are they take the form of thin-walled outgrowths of the body- 

 wall. In some forms in which the surface of the body-wall is 

 increased by the development of a bivalved shell or carapace 

 the lining-surface of the fold serves for respiration, and may 

 be thrown into a number of folds so as to increase the extent 

 of surface, as in the Gasteropod Patella. In the majority of 

 cases, however, more or less branched hollow processes are 

 seated upon the sides of the body or on a greater or less 

 number of the appendages, their cavities communicating with 

 the lacuuar spaces of the body, so that the blood can circulate 

 through them and receive aeration through their thin walls. 

 In the Isopoda a certain number of the appendages are de- 

 voted to the respiratory function, both the exopodite and 

 endopodite being lamellar and thin-walled, or else the endo- 

 podite alone may ha\ r e this function, the exopodite serving as 

 a covering-plate for the protection of the inner respiratory 

 ramus. 



As already stated, the body is covered by a chitinous or 

 more or less calcareous cuticle. This is secreted by the cells 

 of the hypodermis, as it is termed, which correspond to the 

 ectoderm of other forms and rest below on a more or less 

 well-developed layer of connective tissue. A dermal muscu- 

 lar system is entirely unrepresented in the Crustacea, owing 

 no doubt to the development of the thick cuticula ; but never- 

 theless muscles are well-developed. These take the form in 

 the body of four longitudinal bands, two situated dorsally and 

 two ventrally, giving off slips to be inserted into the cuticle 

 of each metamere, flexion and extension of the various meta- 

 meres upon one another being thus permitted. In addition 

 muscles extend from the body-wall to the various appendages 

 and between the various joints of these structures, being in 

 all cases, it is needless to say, situated within the body and 

 the appendages. In some cases, more especially in those 

 forms in which the appendages are adapted for walking, 

 special chitinous plates or processes project into the body- 

 cavity from the ventral surface forming the endophragmal sys- 



