196 



INVERTEBRATE MORPHOLOGY. 



Y'^pkr 



posteriorly to end usually in one or two cereal processes, 

 and anteriorly show a dilatation succeeded by a more or 

 less well-marked narrow region, the two giving rise to a 

 head and neck. The body is covered upon the outside by a 

 cuticle, which may be smooth as in the genus Ichthydium, or 

 take the form of overlapping scales as in Chcetonotus (Fig. 95), 

 sometimes bearing spine-like prolongations. Along the ven- 

 o— /^ tral surface two bands of cilia run from 



the posterior part of the head region 

 almost to the hind end of the body, 

 and in addition to these patches of 

 cilia are found upon the ventral surface 

 and on the sides of the head, some of 

 which are undoubtedly sensory in func- 

 tion. Beneath the cuticular covering 

 lies the ectoderm in the form of a layer 

 of protoplasm in which no cell outlines 

 can be perceived, but which contains 

 numerous scattered nuclei. A pair 

 of longitudinal muscle-bands lie be- 

 neath the ectoderm on the dorsal sur- 

 face, and other bands traverse the 

 ccelom in an antero-posterior direction. 

 Transverse and circular muscles are, 

 however, absent. A distinct ccelom is 

 present, the greater portion of which 

 is occupied, however, by the internal 

 organs ; it is not lined with a peri- 

 toneal epithelium, nor are any mesen- 

 teries present. 



The mouth is situated at the an- 

 terior extremity of the body and opens 

 into a muscular oesophagus (ce), which 

 opens in turn into the cylindrical 

 To this succeeds a short intestine opening to 

 the exterior at the posterior extremity of the body. 



No blood vascular system is present, but the excretory sys- 

 tem consists of a single pair of much-convoluted tubes [nephr) 

 which terminate at one end in a closed ciliated "funnel," 



Fig. 95. — Chcetonotus maxi- 

 mus (after Zelinka). 

 eg = uerve-ganglion. 

 dr = glands. 



i = intestine. 

 m — longitudinal 

 muscles. 

 nephr = nepbridia. 

 06 = cBsopbagus. 

 oil = ovary. 



stomach (i). 



