WORMS. 



217 



filiform, measuring five or six inches in lengtli ; its color is dark asii brown or blackish, 

 a little lighter underneath, and it has three or four eyes in a longitudinal group on 

 each side of the head. (Verrill.) 



A very beautiful and common European species is the Polia crucigera, so called 

 because its dark green body is marked by crosses, made by white stripes and trans- 

 verse rings. Its favorite haunts are calcareous rocks 

 bored out and excavated by other creatures, or else it 

 seeks refuge among the branches of coral, as in our 

 figure, which represents a very perfect specimen 

 drawn life size. 



Tiiere is, perhaps, nothing more interesting in tlie 

 history of the nemerteans than the curious metamor- 

 phoses which some of the forms accomplish, althougii 

 in other cases the development is direct. In the 

 former instance tlie larva passes through the remark- 

 able pilidiuni stage, so called, because the larva was 

 originally described as a new animal under the name 

 of Pilidiuni ; in that condition the embryo is found 



swiniming .about in the ocean; it is almost micro- 



scopic in size, transparent and somewhat like an in- 

 verted bowl in shape, with two fiaps hanging down from the edges, one on each side ; 

 on top a thick mobile hair, the ilagellum ; the edges of the bowl and the two flaps are 

 fringed with delicate cilia to serve, together witii the Hagellum, as locomotive organs. 



Fig. 206. — Piliilii 



Class VIII. — GEPHYEEA. 



The Gcphyrea are an illy-defined group of forms intermediate in many ways 



between the lower worms, i)articularly the nemertean t}pe, and the higher worms 



or annelids proper. Until very recently the Echiurids, which Hatschek's brilliant 



researches have proved to be degraded annelids, were also included in this group, — 



and indeed this im]iortant rectification has not yet found its way into zoological text- 



_ books. The only true gephyreans are those known hitherto 



^ as the Inermes, while the Gephyiei Chajtiferi (Echiuridaj) 



/■ belong to the chjetopods (see Annelida). 



The Gephyrea are not hermajihroditie; they are all 

 marine, usually have a retractile proboscis, and always have 

 a ring of nervous matter round the a'sophagus and a ven- 

 tral nerve cord ; but their bodies are not segmented. The 

 ventral nerve cord distinctly marks an approach to a higher 

 type ; in the lower forms the nervous system is far less con- 

 centrated. The true gephyreans comprise two families, 

 Sipimculida;, having tentacles around the mouth at the end 

 of the proboscis, and PriajJulidiB without tentacles. Of the 

 SiPUNCULiD^E we may consider Phascolosoma typical ; an 

 undeteiTnined European species of this genus is represented 

 in the accompanying figure. Phascoloso7na ca'-mctdarium 

 is very common upon the sandy or shelly bottoms in deep water along all the northern 

 coasts of Xew England. This worm takes possession of a dead shell of some small 



