50 PHYSIOLOGY OF THE INVERTEBRATA. 



They devour their •' Romeos," if the latter are in the least 

 obnoxious. 



(i8) The Arthrogastrct. — In this order, to which the 

 scorpion belongs, " the mouth is situated between the labrum 

 in front, the bases of the pedi palpi and those of the first 

 two pairs of ambulatory limbs at the sides and behind," It 

 is a very small aperture and leads into a pharyngeal sac 

 with chitinous walls. The pharynx passes into a narrow 

 oesophagus, and into this two ducts from large salivary 

 glands discharge the secretion. The intestine forms prac- 

 tically a straight tube which terminates in an anus. As in 

 the Aranci/ia, numerous so-called biliary ducts open into 

 the intestine. The pancreas or digestive gland (the so-called 

 liver) is extremely well developed in Scorpio, occupying all 

 the spaces between " the other organs in the enlarged part 

 of the body, and even extending for some distance into the 

 narrow posterior somites." 



All the Arthrogastrct have a distinctly segmented abdomen. 



(19) The Eurgptcrida form an order of extinct Crustacea, 

 consequently it does not come under our description. 



Before alluding to the other orders, it may be stated that 

 the Crustacea have been subdivided into the Gnathopoda, 

 Pcctostraca, and Malacostraca. The Gnathopoda have been 

 further divided into the Merostomata, Branchiopoda, and 

 Lophgropoda. These three divisions comprise seven orders, 

 commencing with the Eiirgptcrida and ending with the 

 Gopcpoda. 



(20) The Xiphosura. — Of this order the only existing 

 representative is the genus Limulus (the king crabs). The 

 mouth of Limulus is provided with a small labrum, a 

 rudimentary metastoma, and six pairs of lateral appendages 

 which terminate in chela^ It is situated in " the centre of the 

 sternal surface of the anterior division of the body ; the anus 

 opens on the same surface, at the junction between the 

 middle division and the telson." The oesophagus is continued 

 from the mouth forwards and upwards, and then dilates into 



