244 INVBBTEBRATB MORPHOLOGY. 



IV. Class Myzostomeae. 



The Myzostomeae constitute a group of Annelids wliicli pre- 

 sent but few traces of a typical metameric form, being much 

 modified by their parasitic habit. All the known forms are 

 parasitic upon Orinoids, some producing malformations of the 

 pinnules of their host in the form of cysts in the interior of 

 which they live. The body of Myzostomum (Fig. Ill) is 

 flattened and oval, a number of finger-like processes or cirri 

 (c) projecting around the margin. There is no trace of external 

 segmentation, although five pairs of parapodia (p), each with 

 an axial supporting ehitinous rod and a single hooked seta, 

 occur on the ventral surface. On the same surface too, near 

 the margin, are to be found in most species three or four 

 sucker-like depressions (su) on each side, which have been 

 supposed to represent highly-modified nephridia. 



The body is covered by a thick cuticle beneath which lie 

 the hypodermis and the musculature of the body-wall, which 

 has the characteristic Annelidau arrangement. A body-cavity 

 can hardly be said to exist (unless it be indicated by the space 

 occupied by the ova), the interior of the body being completely 

 filled up by the internal organs and by numerous muscle- 

 bands passing both dorso-ventrally and from side to side, 

 these latter in some forms being arranged in such a way as 

 to represent incomplete dissepiments. There is no blood 

 vascular system. 



The mouth is situated near the anterior end of the body on 

 the ventral surface and opens into the proboscis-sheath, 

 within which lies the proboscis {ph), constructed upon the 

 same plan as that of the Ehynchobdellid Hirudinea. Around 

 the extremity of the proboscis are arranged a number of 

 short tentacles, and its walls are very muscular ; behind it 

 opens through a short oesophagus into the wide intestine (s) 

 from which three (or two) branched pouches project on either 

 side towards the margin of the body. The short and relatively 

 narrow rectum (r) opens near the posterior end of the body, 

 uniting shortly before its termination with the oviduct. 



The nervous system consists of a circumcesophageal com- 

 missural ring upon which lie numerous scattered ganglion cells 



