276 CRUSTACEA. 



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. 118). Sense 

 organs are usually well developed, and are not confined 

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

 organs in the tail (Fig. 117). 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 Malacostraca 

 the fore-gut is furnished with a gastric mill. The mid-gut 

 or archenteron is always short, but has connected with it 

 diverticula which form the so-called hepato-pancreas. In 

 the Entomostraca there is usually only a single pair of out- 

 growths ; in Schizopods, 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 are a pair of 

 blind tubes functioning as excretory organs, and presenting 

 an interesting similarity to the Malpighian tubes of insects, 

 which, however, are in connection 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 crelom is virtually absent. In the blood, hsemocyanin 

 is the commonest pigment, but is not universal. Respira- 

 tion 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 Entomostraca 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 pro- 

 bably modified nephridia, and the fact that they open on 

 different segments in the two sexes, is regarded as evidence 

 of the former existence of a series of nephridia like those 

 of Annelids. The process of excretion in the Crustacea 

 is not well understood ; it is possible that shell-making 



