1817.] ANATOMICAL DESCRIPTIONS. 185 



mal. On one of t!ie sides of the stomach, one distin- 

 gmshes a small expansion, the object of which organiza- 

 tion I have not been enabled to discover. 



The crust whereof tMese animals form their habita- 

 tions, appears to be constructed of successive layers of 

 sand, the base of which is augmented in proportion as the 

 animal advances in its labours. 



This species is that which approaches nearest to the 

 madrepores, not only in its external form, but in the per- 

 manency of its envelope^ and its large expansion. 



i^fter the examiniitio» that I have made of the animals 

 above mentioned, we perceive that they are furnished 

 with a skin or covering, more or less susceptible of con- 

 traction or dilatation, enclosing an organization construct- 

 ed on the same system, but modified, according to theirpar- 

 ticular habits. We perceive that they are all provided with 

 tentacula, analogous to ariiis, fit to seize and retain their 

 prey; with an alimentary sack, open at both extremities, 

 surrounded at its base with several arcuated organs, which 

 may be considered as performing the functions of the liver. 

 The membranes which intersect the interior of the body^ 

 may be equally compared to gills, which receive the water 

 admitted by the two openings of the stomach; this water 

 may pass out through the same openings, or by the exte- 

 rior tubercles, as in die .^. olivacea, and the yi. ultrama- 

 rina^ or at the extremity of the tentacula, as in the A. flava. 

 The white filaments which support the interior mem- 

 branes, are analogous to those considered as nerves, by 

 Dr. Spix, in the A, coriacea of Europe. (Ann. des Mu- 

 seum, tome 13; p. 443.) These nerves have equally their 

 origin in the base of the animals described above. It is 

 easy to conceive that the stomach, haviiig apertures at its 



