XI 



PHYLUM ARTHROPODA 537 



The digestion of the food and to some extent the absorption of 

 the digested products are performed by a pair of large glands (lr.), 

 lying one on each side of the gizzard and anterior end of the 

 intestine. They are formed of finger-like sacs or cceca, which 

 discharge into wide ducts opening into the mid-gut, and are lined 

 with glandular epithelium derived from the endoderm of the embryo. 

 The glands are often called livers, but as the yellow fluid they 

 secrete digests proteids as well as fat, the name hepato-pancreas 

 is often applied to them, or they may be called simply digestive 

 glands. The Crayfish is carnivorous, its food consisting largely 

 of decaying animal matter. Microscopic glands occur in the 

 wall of the gullet. 



The digestive organs and other viscera are surrounded by a 

 body -cavity, which is in free communication with the blood-vessels 

 and itself contains blood. As will be pointed out more particularly 

 hereafter, this cavity is to be looked upon as an immense blood- 

 sinus, and not as a true coelome. 



There are well-developed respiratory organs, in the form of 

 gills , contained in a narrow branchial chamber, bounded internally 

 by the proper wall of the thorax (Fig. 446, ep), externally by the 

 gill-cover or pleural region of the carapace (led). Each gill con- 

 sists of a stem giving off numerous branchial filaments, so that 

 the whole organ is plume-like. The filaments are hollow, and 

 communicate with two parallel canals in the stem an external, 

 the afferent branchial vein, and an internal, the efferent branchial 

 vein. The gill is to be considered as an out-pushing of the 

 body-wall, and contains the same layers a thin layer of chitin 

 externally, then a single layer of epithelial cells, and beneath this 

 connective-tissue, hollowed out for the blood channels and con- 

 taining gland-cells, which will be referred to presently (p. 539). 



According to their point of origin, the gills are divisible into 

 three sets first, podobranchice or foot-gills, springing from the 

 epipodites of the thoracic appendages, from which they are only 

 partially separable ; secondly, arthrobranchice or joint-gills, spring- 

 ing from the articular membranes connecting the thoracic 

 appendages with the trunk ; and thirdly, pleurobranchice, or 

 wall-gills, springing from the lateral walls of the thorax, above the 

 attachment of the appendages. It is inferred from the study of 

 other Crayfishes, that a typical thoracic segment bears four gills, 

 one podobranch, two arthrobranchs, and one pleurobranch. But 

 in Astacus one or more of the gills in every segment are absent or 

 vestigial, and the table, or " branchial formula," on page 538 shows 

 the actual number and arrangement of these organs, ep standing for 

 epipodite, and r for the vestige of a gill. 



By adding up the columns vertically we get the number of gills 

 in each segment ; ' by adding them horizontally, the number of each 

 kind of gill ; and by adding together the results obtained by both 



VOL. i. M M* 



