52 



INTRODUCTION TO CLASSIFICATION. 



thought it probable that these " pseudo-haemal " vessels are 

 extreme modifications of organs homologous with the water- 

 vessels of the Scolecida. As M. cle Quatrefages has clearly 

 shown, it is the perivisceral cavity with its contents that, in 

 these animals, answers to the true blood-system of the Crustacea 

 and Insects. 



The embryos of Annelids are very generally ciliated, and 

 Fig. 26. vibratile cilia are commonly, if not universally, 

 developed in some part or other of their organiza- 

 tion. In both these respects they present a most 

 marked contrast to the succeeding classes. 



XIX. THE CHJETOGNATHA. 



There is a very aberrant marine genus, Sa- 

 gitta, the nearest affinities of which appear to be 

 with the Annelida, but which is so unlike them 

 and every other group as to require a class for 

 itself. 



The Sagittse are elongated and transparent 

 animals, with rounded heads and tapering caudal 

 extremitjes, which do not usually attain much 

 more than an inch in length. 



The head is provided with several, usually six, 

 sets of strong bilaterally symmetrical oral setae, 

 two of which, long and clavv T like, lie at the sides 

 of the mouth ; while the other four sets are short, 

 and lie on that part of the snout which is pro- 

 duced in front of the oral aperture. The posterior 

 part of the body is fringed on each side by a 

 delicate striated fin-like membrane, whicli seems 

 to be an expansion of the cuticle. In some species 

 Fig. ze.Sagitta fa e body is beset with fine setae. The intestine 



bipunctata, seen * . 



from below, a, the is a simple, straight tube, extending irom the 



an? ^endaS 6 ; m Utl1 tO the anUS 5 the ktt<3r O P 6nS n the V 6ntral 



&, the anus" c, surface, just in front of the hinder extremity. A 

 testicuiar ' ch'am- single oval ganglion lies in the abdomen, and 

 ber f- sends, forwards and backwards, two pairs of lateral 



?- of 



