376 INVERTEBRATE MORPHOLOGY. 



tern and serving for the attachment of the muscles passing to 

 the appendages. In forms furnished with a bivalved shell 

 special adductor muscles for its closure are frequently devel- 

 oped ; and in the higher Crustacea, in which the so-called 

 stomach is usually provided with a series of chitinous teeth, 

 special muscles are developed for their movement. 



The coelom of the Crustacea consists for the most part of 

 a series of cavities, without definite walls, between the viscera 

 and the muscle-bundles and extending out into the append- 

 ages and the branchiae. One of these occupies the mid-line 

 below the dorsal surface of the body, and contains the heart, 

 whence it is known as the pericardial sinus. It is bounded 

 below by a .distinct partition, the pericardial septum, but 

 seems to be a schizocoelic space, since it contains blood, and is 

 therefore not comparable to the pericardial cavity of the 

 Mollusca. A true enteroccel does exist, however, in some of 

 the higher forms (e.g., Palcemonetes), consisting of a sac lying 

 in the anterior thoracic region. It surrounds the anterior 

 aorta as a narrow cavity and behind expands so as to cover 

 the anterior portion of the reproductive organs, and then 

 passes veutrally into the schizocoelic cavity which surrounds 

 the intestine. It is a perfectly closed sac, having no com- 

 munication with the pericardial sinus beneath which it lies, 

 and contains a coagulable fluid in which no corpuscles have 

 been observed. 



The saclike cavity into which the antenuary gland, to be described 

 later, opens is also to be regarded as a true enterocosl; but attention must 

 again be called to the inadvisability of maintaining a wide distinction be- 

 tween schizoccelic and enterocoelic spaces. (See p. 231.) 



The circulatory system is comparatively simple. In many 

 forms a heart and distinct blood-vessels are entirely wanting, 

 the blood circulating through the lacunar coelom by the 

 movements of the appendages. In the majority of forms, 

 how r ever, a pulsatile heart (Fig. 168, /*) is present, lying near 

 the dorsal surface of the body in the pericardial sinus, ex- 

 tending in some forms throughout the entire thoracic and 

 abdominal regions of the body. More usually, however, the 

 heart is limited to the thoracic region, or occasionally is 

 almost entirety confined to the abdomen, its anterior ex- 



