XI 



PHYLUM ARTHROPODA 



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The mouth leads into a narrow gullet (gul.), which passes upwards 

 and forwards into the head and enters a wide stomach (st.), from 

 which a straight intestine (int.) is continued back to the terminal 

 anus (an.) From each side of the stomach is given off a wide 

 tube (d.gl.) which branches extensively, its ramifications finally 

 ending in delicate caeca. The larger branches of these digestive 

 glands contain food in process of digestion : their ultimate cseca 

 secrete a digestive juice : the walls of the stomach itself are non- 

 glandular. The walls of the enteric canal consist of an inner layer 

 of epithelium and an outer layer of connective-tissue and muscle. 

 In the gullet and in the posterior end of the intestine the epithelium 

 secretes a thin cuticle, which thus comes to form the actual lining 

 of the cavity. It is shown by development that the portion of the 

 canal devoid of a chi- .. 



tinous lining is formed 

 from the archenteron 

 of the embryo : the 

 gullet is developed 

 from the stomodaaum, 

 the posterior end of 

 the intestine from the 

 proctodseum. 



The body-cavity is 

 divided into several 

 parts by membranous 

 partitions (Fig. 432) : 

 there is a large median 

 cavity in which the 



enteric Canal (*') lies, FIG. 432. Transverse section of Apus. cm. muscles to feet ; 



11 I . , -I dv. dorso-ventral muscles : e. eggs ; dm. dorsal muscles ; 



called tne intestinal g. ovary; h. heart; i. intestine; m. partition between 



sinus : on each side of 2Bj lateral Sinus ; ' ventral musdes ' (Fr m 



this are lateral sinuses 



containing the muscles ; and in the dorsal region is a median 



cavity, the pericardial sinus. All these spaces are devoid of an 



epithelial lining, and contain blood : as will become evident 



later, they do not correspond with the coelome of the higher 



worms. 



The central organ of the circulatory system is the heart (Fig. 

 431, ht, and Fig. 432, h), a narrow tube contained in the pericardial 

 sinus. It is pierced laterally by several pairs of apertures or ostia 

 provided with valves opening inwards, and is continued in front 

 into a narrow tube, the cephalic artery (c. art.), which extends into 

 the head and gives off near its origin a pair of arteries to the shell- 

 glands (Fig. 429). When the heart contracts, the blood is driven 

 through these arteries to the head and carapace : it then travels 

 backwards in the intestinal sinus, passes to the limbs, and is 

 returned to the pericardial sinus, finally re-entering the heart, 



