oc6 PHYLUM ARTHROPODA 



sented by a lobed ganglionic mass in the thorax, connected 

 with a mere rudiment, which corresponds to the abdominal 

 portion of the cord in the crayfish (Fig. 199). Sense 

 organs are usually well developed, and are not confined 

 to the head region ; thus many Mysids have " auditory " 

 organs in the tail (Fig. 198). The alimentary canal 

 runs straight throughout the body ; it consists of fore-gut, 

 mid-gut, and hind-gut. The fore-gut and hind-gut are 

 anterior and posterior invaginations of ectoderm, and are 

 always large, especially in Malacostraca. In the higher 

 Malacostraca the fore-gut is furnished with a gastric mill. 

 The mid-gut or archenteron is always short, but has con- 

 nected with it diverticula which form the so-called hepato- 

 pancreas. In the Entomostraca there is usually only a 

 single pair of outgrowths ; in Mysids, Cumacea, and larval 

 Decapods there are three pairs ; a process of rapid growth 

 and branching converts these into the compact digestive 

 gland of the adult Decapods. In connection with the 

 posterior end of the mid-gut in Amphipods and some 

 others, there is a pair of blind tubes functioning as excre- 

 tory organs, and presenting an interesting similarity to the 

 Malpighian tubes of insects, which, however, are in con- 

 nection with the hind-gut. The body cavity is never large, 

 being mainly filled up with muscles and organs, and, as in 

 Arthropods in general, the true coelom is virtually absent. 

 In the blood, haemocyanin is the commonest pignient, but 

 is not universal. Respiration is carried on in many 

 different ways. In the simple forms it may be merely by 

 the general surface, but in the majority of cases, certain 

 portions of the limbs, or outgrowths of the limbs, constitute 

 definite respiratory organs, often specialised to form gills. 

 In the excretory system the numerous nephridia of Annelids 

 are absent. The typical excretory organs of the Entomos- 

 traca are the " shell glands " — paired coiled tubes opening 

 on the second maxilla ; of the Malacostraca, the antennary 

 glands exemplified by the green glands of the crayfish. 

 The genital ducts are possibly modified nephridia. 



There are many peculiarities connected with reproduc- 

 tion — 'thus parthenogenesis for prolonged periods is 

 common among '* water-fleas " ; hermaphroditism is 

 frequent, occurring, for example, in barnacles, acorn- 



