LESS, xxvn CIRCULATORY ORGANS 337' 



pdb)) springing from the epipodites of the thoracic appen- 

 dages, from which they are only partially separable ; secondly, 

 arthrobranchia or joint-gills (B, arb\ springing from the 

 articular membranes connecting the thoracic appendages 

 with the trunk; and thirdly, pleurobranchice, or wall-gills 

 (plb\ springing from the lateral walls of the thorax, above 

 the attachment of the appendages. The total number of 

 gills is eighteen, besides two filaments representing vestigial 

 or vanishing gills. 



The excretory organs differ both in position and in form 

 from those of Polygordius. There are no distinct nephridia, 

 but at the base of each antenna is an organ of a greenish 

 colour, the antennary or green gland (Fig. 83, A, K), by 

 which the function of renal excretion is performed. The 

 gland is cushion-shaped, and contains canals and irregular 

 spaces lined by glandular epithelium : it discharges its secre- 

 tion into a thin-walled sac or urinary bladder , which opens 

 by a duct on the proximal segment of the antenna. The 

 green glands are to be looked upon as organs of the same 

 general nature as nephridia. 



The circulatory organs are in a high state of development. 

 The heart (Fig. 83, Ht ; Fig. 86, ti) is situated in the dorsal 

 region of the thorax, and is a roughly polygonal muscular 

 organ pierced by three pairs of apertures or ostia (Fig. 86, o\ 

 guarded by valves which open inwards. It is enclosed in a 

 spacious pericardial sinus (Fig. 83, Pcd. S\ which contains 

 blood. From the heart spring a number of narrow tubes, 

 called arteries, which serve to convey the blood to various 

 parts of the body. At the origin of each artery from the 

 heart are valves which allow of the flow of blood in one 

 direction only, viz., from the heart to the artery. From the 

 anterior end of the heart arise five vessels a median 

 ophthalmic artery (Fig. 86, oa), whicli passes forwards to the 



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