THE POLYCILETA. 207 



the anterior region of the body. They very generally have a 

 proboscis, provided with chitinous teeth. 



The singular genus, Tomopteris, is a transparent pelagic 

 Annelid, with numerous parapodia, each terminated by two 

 lobes representing the neuropodium and notopodium, but 

 with setae, two of which are very long, only in the cephalic 

 region. 



The sedentary Annelids (Tubicola) fabricate tubes, either 

 by gluing together particles of sand and shells, or by secret- 

 ing a chitinous or calcined shelly substance, in which they 

 remain (e. g\, Protula, Fig. 54). The praestomium is small or 

 wanting ; none have a proboscis ; there are no cirri ; and the 

 parapodia are short or rudimentary. The branchias are devel- 

 oped only on the anterior somites, and the latter are often 

 markedly different from those which constitute the posterior 

 part of the body. 



In some (Serpulidoe) a tentacle is enlarged and its end 

 secretes a shelly plate which serves as an operculum, and 

 shuts down over the mouth of the calcareous tube inhabited 

 by the animal, when it is retracted. The dilated end of the 

 opercular tentacle sometimes serves as a chamber in which 

 the young undergo their development (species of Spiro7 , bis). 



The alimentary canal of the polychsetous Annelida rarely 

 presents any marked distinction between stomach and intes- 

 tine, and is almost always of the same length as the body, ex- 

 tending, without folds or convolutions, from its anterior to 

 its posterior extremity ; but in Sip>honostomum (Chlorcema), 

 Pectinaria and others, it is more or less convoluted. It is 

 attached by membranous bands, or more complete mesenteries, 

 to the walls of each somite, and very commonly presents a dila- 

 tation between every pair of mesenteries. In most Polychoeta, 

 the intestine acquires in this way merely a moniliform appear- 

 ance, but in Polynoe, Aphrodite, Sigalion, and their allies, 

 long caeca are given off upon each side of the alimentary 

 canal, and, sometimes becoming more or less convoluted, ter- 

 minate at the upper part of each segment (Fig. 51, D) close 

 beneath, or in the branchiae, w r here such organs exist. 



The anterior portion of the alimentary canal is, in a great 



number of the Polychmta, in fact in all the typical Errantia, 



so modified as to constitute a distinct muscular pharynx, the 



anterior portion of the wall of which can be everted like the 



finger of a glove, from the aperture of the mouth, and the 



posterior portion protruded, so as to form a proboscis. In 



Polynbe sqi/amata, the proboscis is one-fourth as long as the 

 14 



