39.2 THE ANIMAL PARASITES OF MAN 



The INTESTINAL CANAL usually consists of the anterior, median and 

 terminal intestine, and as a rule passes straight through the body ; a 

 number of salivary glands discharge into the anterior part, and, in some 

 cases, yield a stiffening secretion which serves for spinning webs ; numerous 

 or scanty hepatic tubes are appended to the median intestine, while on 

 the border between the median and terminal intestine open four to six 

 long tubes (vasa malpighiana), which act as urinary organs. Finally the 

 end portion of the intestine carries various glands (anal and rectal glands, &c.) 



The CENTRAL NERVOUS SYSTEM agrees in structure with that of the 

 annelids, but is more highly developed. The pharyngeal ring surrounds 

 the front part of the intestine ; the sensory nerves originate from its 

 SUPERIOR PHARYNGEAL GANGLIA and are the seat of the higher psychical 

 functions ; the INFERIOR PHARYNGEAL GANGLIA govern the mouth parts, 

 and in addition appear to regulate the movements (cerebellum of the 

 vertebrates). 



The chain of GANGLIA lying on the ventral side of the abdomen con- 

 sists primitively of pairs of ganglia corresponding to the twelve segments, 

 which are connected by longitudinal and transverse commissures. But 

 many changes in the ganglia may be seen in insects caused by partial or 

 entire amalgamation of single ganglia, so that in a few cases only one 

 abdominal ganglion is present. In conclusion, a definite INTESTINAL NER- 

 VOUS SYSTEM is always present. 



Of the organs of sense the FACETTED EYES, situated at the sides of the 

 head, deserve special mention, as do also the ORGANS OF TOUCH and SMELL, 

 situated on the antennae, and the ORGANS OF HEARING and taste, or finer 

 sensations, situated at the mouth and in the buccal cavity. 



The sounds emitted by insects are as a rule produced by the friction 

 or beating of certain chitinous parts, but sounds are also produced in 

 breathing (flies). 



The ORGANS OF RESPIRATION, the so-called tracheae, are highly developed ; 

 there are openings (stigmata) at the sides of the body which draw in air 

 by means of the active participation of the muscles of the body. The 

 number of stigmata varies between two and ten pairs ; the tracheae them- 

 selves branch off from the trunks in the most varied manner, and carry 

 air to the internal organs. 



The colourless BLOOD circulates between the tissues and organs, and is 

 kept circulating by the contraction of a chambered dorsal vessel provided 

 with ostia, and which terminates in a short aorta opening at the anterior 

 end. 



Insects are SEXUALLY DISTINCT ; their sexual glands are in pairs and 

 have a tubular structure, but the testicular tubules are united together 

 by a capsule into an oval testicle ; exceptionally, also, the excretory canals are 

 double, as also the sexual orifices ; usually the paired canals unite into a single 

 oviduct or spermatic duct which terminates at the posterior end of the body 

 after receiving the products of various glands. 



As to the HISTORY OF THE DEVELOPMENT of insects, all that is necessary to 

 mention here is that the young hatched from eggs only exceptionally (as in 

 apterygota) resemble the ackilt parent (insecta ametabola) ; as a rule they 

 differ from them not only in the shape of the body, but also more or less 

 by their manner of life, and only attain the form of the parent through 



