THE DECAPODA 283 



there ai'ises anteriorly in Astacura and most Thalassinidea a short 

 unpaired caecum. In CallidiMs.^a among the Thalassinidea and in 

 most Paguridea a pair of longer or shorter caeca are present, aiid 

 in most Brachyura they form two long and convoluted tubules. In 

 the Caridea and Galathaeidea and in Fagnristes (Paguridea) no 

 caeca are found. Dromia possesses a single short caecum, and so 

 resembles the Astacura and differs from the other Brachyuia. 



An unpaired caecal tube of considera1)le length springs from 

 the dorsal surface of the intestine in the Brachyura, and a shorter 

 caecum is present in the Lobster, in the Thalassinidea, and in some 

 Paguridae. It is probable that in all these cases the caecum arises 

 from the posterior end of the mid-gut. In Aljpliem, according to 

 Coutiere, the mid-gut is produced backwards beyond its junction 

 with the narrower hind-gut into a number of blind saccules. 



Groups of gland-cells on the Avails of the oesophagus, on up})er and 

 lower lips, and on the maxillulae and maxillae, have been regarded 

 as salivary glands. C^uite similar glaiids, however, may occur through- 

 out the whole length of the hind-gut also, and they are identical in 

 structure with the dermal glands which occur in various situations 

 on the surface of the body. 



With the single exception of Leucifer, which i)ossesses only two 

 pairs of hei«itic caeca, the vobmu'nous " liver " of the Decapods 

 consists of a mass of miiuitel}' ramified tubules, lying mainly in the 

 thorax. It communicates with the anterior part of the mid-gut by, 

 as a rule, a single duct on each side, but in Alphe^is (Coutiere) three 

 ducts are present. In Paguridae the hepatic glands are displaced 

 backwards, and lie for the most part in the abdominal region. 



Circulatory Si/stem. — The heart in all Decapods is short, 

 polygonal in outline, and situated under the posterior part of 

 the carapace. As a rule there are three pairs of venous ostia, 

 of which one, or in the Brachyura two pairs are situated on the 

 upper surface. Coutiere has demonstrated the existence of two 

 additional pairs in certain Caridea, and possibly further research 

 will show that these are present in other cases. 



Anteriorly the heart gives off a median ophthalmic artery 

 which runs forward to supply the region of the eyes. On each 

 side of this originates an antennal artery, which, besides supplying 

 the antennae, sends branches also to the rostrum, eyes, and adjacent 

 parts. In Astacus, Bouvier finds that terminal branches of the 

 antennal arteries unite in front of the brain in a median vessel 

 which runs backwards to anastomose on the walls of the oesophagus 

 Avith branches of the sternal (subnenral) artery — an arrangement 

 which recalls the circumoesophageal vascular ring of some Isopoda 

 and Amphipoda. A second pair — the hepatic arteries — arise from 

 the sides of the heart a little way behind the antennal arteries, 

 and are distributed to the hepatic glands and adjacent viscera. 



