40 PHYSIOLOGY OF THE IXVERTEBRATA. 



The Sagitta have rouuded, elongated, unsegmented bodies. 

 There is a head, at the anterior end of which is the mouth. 

 On each side of the mouth is situated several strong, curved, 

 chitinous spines, which are said to act as jaws. The mouth 

 passes into a straight alimentaiy canal which terminates in 

 an anus situated ventrally and anteriorly to the caudal 

 region. The caudal region ends in a fan-like " fin " of 

 delicate set^. No salivary glands or pancreatic follicles 

 appear to exist in Scif/itfa. The endoderm of the alimentary 

 canal possibly performs the function of a digestive gland. 



The genus Sagitta includes several species of vermiform 

 animals which live near the surface of " the ocean in all parts 

 of the world." 



The Arthropoda. 



This is the third and last class of the Arthrozoic Series, and 

 is divided into the Onydioplwra, Myriapoda, Insecta, Arach- 

 nida, and Crustacea. These divisions are again subdivided 

 (see the table given in Chapter I.). 



(i) The Prototrachcata. — The only genus is Pcripatus^ and, 

 due to the important investigations of Professor H. N. Moseley, 

 F.R.S,* Pcripatus has been referred to the Artlirojjoda. All 

 the species of this genus have a well-developed tracheal 

 system. There is a distinct head, with several tentacula; 

 the mouth is situated ventrally beneath a large projecting 

 suctorial lip, and is provided with a pair of mandibles or 

 jaws. There is also a short oral papilla attached to the head 

 on each side of the mouth. Tlie mouth or oral aperture 

 leads into a muscular pharynx ; then follows a short oeso- 

 phagus, which passes into a wide and long stomach. The 

 stomach leads into a short intestine terminating in an anus 

 situated at the posterior end of the body. There appears to 

 be no salivary glands or pancreatic follicles. 



(2) The Chilopoda. — On referring to the classification given 

 in the first chapter of the book it will be observed that the 



* PliUosopltical TrcDif^actioiis of the lioyaJ Society, 1874. 



