WOBMS. 217 



filiform, measuring five or six inches in length ; its color is dark ash browu 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. 



There is, perhaps, nothing more interesting in the 

 history of the nemerteans than the curious metamor- 

 phoses which some of the forms accomplish, although 

 in other cases the development is direct. In the 

 former instance the larva passes through the remark- 

 able pilidium stage, so called, because the larva was 

 originally described as a new animal under the name 

 of Pilidium ; in that condition the embryo is found 

 swimming about in the ocean; it is almost micro- FiG.206.-pmdiumcbnta^ini„g(^^^ 

 scopic in size, transparent and somewhat like an in- 

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

 on top a thick mobile hair, the flagellum ; the edges of the bowl and the two flaps arc 

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



Class YIII. — GEPHYREA. 



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

 between the lower worms, particularly the nemertean type, and the higher worms 

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

 researches have proved to be degi-aded annelids, were also included in this group, — 

 and indeed this important rectification has not yet found its way into zoological text- 

 books. The only true gephyreans are those known hitherto 

 as the Inermes, while the Gephyrei Chsetiferi (Echiuridse) 

 belong to the chsetopods (see Annelida). 



The Gephyrea are not hermaphroditic; they are all 

 marine, usually have a retractile proboscis, and always have 

 a ring of nervous matter round the oesophagus 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, 

 Sipunculidaj, having tentacles around the mouth at the end 

 of the proboscis, and PriapulidiB without tentacles. Of the 

 SiPUNCULiD^ we may consider Phascolosoma typical ; an 

 undetermined European species of this genus is represented 



Fig. 207. — Phascolosoma. . ^ -r. 7 ? j • 



in the accompanying figure. Phascolosoma cmmentarmm 

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

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



