218 



LO WE It INVER TEBRA TES. 



gastropod, like the hermit crabs, but as the aperture is always too large for its bo<ly, 



it builds out the rim of the aperture until only a small round opening is left, througli 

 which the worm can stretch fortii the anterior extremity of its 

 body, 01' withdraw into the shell it has appropriated for its dwell- 

 ing. It lives permanently in its borrowed home, dragging it about 

 by the powerful contractions of its body. The material with which 

 the Phascolosoma patches out the shell so as to constrict tlie 

 mouth thereof, is a hard and durable composition of sand and 

 mud, cemented together by a secretion of the animal. When 

 fully extended the forward jjart of the body, or so-called proboscis, 

 is long and slender, and furnished close to the end with a circle of 

 small slender tentacles, which surround the mouth. There is a 

 band of minute spinules just behind the mouth. When tlie worm 

 grows too large for its habitation, instead of 

 exchanging it for a larger shell, as do the 

 hermit crabs, it gradually extends its tube 

 outward from the mouth of the shell, a labor 



Fic jub - I'Un^coioio- sometimes rudely interruijted by a lisli swal- 

 lowing the worm, shell and all. 

 Priapulus, of which we give an admirable figure, is the type 



of the second family, the Peiapulid,*;. Even its external form 



indicates the necessity of placing it apart from Sijjunculids. 



The anterior club-shaped portion of the body is the jiroboscis 



with numerous dentate ridges. The tail is covered with nu- 

 merous round i)apill;e. The genus is common in deep water 



along the shores of northern Europe, and burrows a hole in sandy 



bottoms, where it resides as happily as the Phascolosoma in its 



stolen shell, and leads an equally uninteresting life. 



The genus Pkoronis stands quite apart, an<l in fact has been 

 classed with the annelids by some writers, but our present know- 

 ledge indicates rather affinities with the gephyreans. It lives 

 in tubes on the sea bottom, and has a crown of tentacles around 

 the mouth. Its larva, known as Actinotrocha, is a remarkable 



creature, with long ciliated arms about its body, and changing into the adidt worm by 

 turning itself apparently inside out. 



Class IX. — ANNELIDA. 



We j)ass now to the last of the worms in the ascending series, the large and varied 

 class of segmented or jointed worms, which includes the leeches, the common eartii 

 worms, and an immense number of marine, fresh water and teri-estrial sjjecies, which 

 attract the naturalist by the wonders of their organization, development and habits. 

 Every sort of life which is open to crawling and swimming creatures is led by one 

 or another of the annelids ; some grovel in the eartli or live in mud ; some take refuge 

 amid rocks and corals; some wander freely over the ocean, and are gaily decorated ; 

 others are mean parasites, feeding and dwelling on larger creatures, and leading 

 greedy, selfish lazy lives, like human criminals, stealing the property of otheis. 



