THE BLOW-FLY. 35 



freely in the cavity thus formed. At the base of the cavity four 

 long caecal appendages arise, the homologues of those which in 

 greater numbers and much smaller in size surround the proven- 

 triculus of Tcviypus. The distribution of the tracheae in this organ, 

 as in other parts of the alimentary canal, may be seen to great 

 advantage in the freshly killed insect. The superficial tracheae are 

 shown in Fig. 17, but others dip down into the space between the 

 termination of the oesophagus and its reflected wall. The proven- 

 triculus is followed by the ventriculus or stomach, an organ of 

 considerable length extending to the insertion of the bile tubes. 

 Here the epithelial lining is very distinct, the cells being about 

 i-3ooth of an inch in diameter, and filled with granular contents. 

 The function of these cells is doubtless the secretion of a gastric 

 juice for the purpose of digestion. From the insertion of the 

 bile-tubes, a long-coiled intestine of less diameter than the stomach 

 extends to the anus ; here the epithelial wall is very little developed, 

 but a coating of longitudinal and circular muscular fibres is very 

 clearly seen, and here probably absorption chiefly takes place. 



In connection with the alimentary canal we find the salivary 

 glands (Fig. 5) and the bile-tubes (Fig. 6); the former open by a 

 common ringed duct, indistinguishable from a trachea, into the 

 pharynx ; they are lined with nucleated epithelium, i -400th of an 

 inch in diameter, and are sausage-shaped, their extremities being 

 bound together by a mass of fatty rete. The bile-tubes are four 

 in number, uniting in pairs just previous to their insertion at the 

 extremity of the stomach. 



The pharynx has been referred to lately in our note-books as 

 the skull. I think it will be admitted that a skull ought to have 

 in it something of the nature of brain, but inasmuch as the great 

 nerve-centres of the larva which represent its brain lie wholly 

 outside this organ, we must take it, that this nomenclature is 

 scarcely applicable. Both in the larva and in the perfect insect 

 this organ is a sucking instrument, and its construction is essentially 

 the same. The horny parts (Figs. 7, 8, and 9) of which it is 

 composed are extended backwards in the form of four processes, 

 and they enclose the commencement of the oesophagus and a pair 

 of internal transverse muscles ; they are moreover provided with 

 powerful external muscles. From a consideration of Fig. 9, it will 



