XI 



PHYLUM ARTHROPODA 533 



paired dorsal (d.m.) and ventral bands, which pass respectively 

 above and below the origins of the limbs : the dorsal bands arise 

 in front from the head-region, the ventral from a strong fibrous 

 plate, the cephalic apodeme (c.ap.), lying just behind the gullet. 



Each appendage is moved as a whole by muscles passing into it 

 from the trunk: its various parts are acted upon by delicate 

 muscular slips running to the various podomeres of the axis and 

 to the endites, thus rendering them separately movable. The only 

 example we have yet met with of appendages moved by definite 

 muscular bands is that of the curious Rotifer Pedalion (p. 330). The 

 muscles are all striped, a character which applies to the Arthropoda 

 generally, with the exception of the Onychophora. 



Digestive Organs. The mouth (Fig. 424, mth.) is situated on 

 the ventral surface of the head, and is bounded in front by the 

 labrum (Ibr.), on each side by the mandibles, and behind by the 

 paragnatha. The food appears to be pushed forwards towards 

 the mouth by the toothed bases of the thoracic feet, and is 

 broken up by the mandibles, which work laterally. The maxillae 

 are probably functionless, or nearly so. 



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 exten- 

 sively, its ramifications finally ending in delicate caeca. The 

 larger branches of these digestive glands contain food in process of 

 digestion : their ultimate caeca 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 chitinous 

 lining is formed from the archenteron of the embryo : the gullet is 

 developed from the stomodaeum, the posterior end of the intestine 

 from the proctodseum. 



The body-cavity is divided into several parts by membranous 

 partitions (Fig. 425) : there is a large median cavity in which the 

 enteric canal (i) lies, called the intestinal sinus : on each side of this 

 are lateral sinuses containing the muscles : and in the dorsal region is 

 a median cavity, the pcricardial sinus. All these spaces are 

 devoid of an epithelial lining, and contain blood: as will become 

 evident later (cf. p. 593), they do not correspond with the coelome 

 of the higher worms. 



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

 424, ht, and Fig. 425, h), a narrow tube contained in the pericardial 

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



