PHYLUM TROCHELMINTHES 



337 



Aimulata a phylum to be treated of later. The Dinophilea are free-living animals. 

 mostly marine, one species living in brackish water. The Histriobdellea are 

 parasitic or commensal, and live on the European lobster and the Australian 

 fresh-water crayfishes. 



Dinophilus (Fig. 274) is a minute worm-like animal with a head or pro- 

 stomium, a body composed of from five to eight segments separated from one 

 another by constrictions, and a short ventral tail. The prostomium bears two 

 eye-spots and some sensory hairs : it is either covered uniform!}' with cilia, or 

 bears two or three annular ciliated bands apparently representing the prototroch 

 of the trochophore. The body is in some of the species uniformly ciliated ; in 

 others the cilia are disposed in rings corresponding to the segments, except on 



Fi<;. -74. Dinophilus taeniatus. The left figure represents the dorsal surface of a young 

 individual, x 76 ; the mouth and alimentary tract are seen by transparency. The right 

 figure shows the anatomy of the male, x 38. an. anus ; 6. rectum ; c. body-cavity ; d. vas 

 deferens ; in. pharynx ; n'. the first nephridium ; ?. entrance to the oesophagus ; p., in left 

 fig., prostomium; p., in right fig., penis; st. stomach; s. x. vesiculaj seminalis. (From 

 Sheldon, after Harmer.) 



the ventral surface, where the ciliation is always uniform. The mouth, which is 

 situated on the ventral aspect of the prostomium, leads into an alimentary canal 

 consisting of oesophagus, stomach, and intestine, all of which are ciliated ; the 

 anus (an) is placed dorsally over the tail. A protrusible muscular proboscis lies, 

 when retracted, in a recess opening close to the mouth. There is an imperfectly 

 developed ccelome which is crossed by strands of connective tissue. A nervous 

 system is present, and consists of a large dorsal ganglion in the prostomium, 

 giving off two anterior, and two posterior nerves or ventral cords (sometimes 

 segmented into a series of ganglia connected in each segment by commissures), 

 all situated in the epidermis. 



The excretory system consists of a series of metamerically arranged pairs of 



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