ACEPHALA TESTACEA. 283 



GASTROCH^NA, Spengler. 



Where the shells are deprived of teeth, and their edges being wide apart 

 anteriorly, leave a large oblique opening, opposite to which there is a small 

 hole in the mantle for the passage of the foot. The double tube, which can 

 be retraced completely within the shell, is susceptible of being greatly elon- 

 gated. It appears that they are certainly furnished with a calcareous tube. 

 They inhabit the interior of Madrepores, which they perforate. 



Two genera of Acephala furnished with tubes, have been de- 

 tected among fossils, but the first of them, the 



TEREDINA, Lam., 



Has a little cuilleron on the inside of each of its valves, and a small, free 

 shield-shaped piece on the hinge. In the second, 



CLAVAGELLA, Lam., 



One of the valves is clasped by the tube, leaving the other, however, free. 

 A single living species is found in the Madrepores of the Sicilian seas, which 

 has been described by M. Audouin. 



Some naturalists think we should also place in this family the 



ASPERGILLTJM, Lam. 



The shell of which is formed of an elongated conical tube, closed at its 

 widest extremity by a disk perforated with numerous small tubular holes; 

 the little tubes of the outer range being longest, form a kind of corolla 

 round it. The reason for approximating them to the Acephala with tubes 

 is found in the fact that there is a double projection on one part of the cone 

 which really resembles the two valves of the Acephala. The affinity be- 

 tween these little tubes and those which envelope the tentacula of certain 

 Terebella, formerly caused this animal to be referred to the Annulata. 



The species most known, Jlsper. javanum, is seven or eight inches in 

 length. 



ORDER II. 

 ACEPHALA NUDA. 



The naked Acephala are not numerous, and are sufficiently re- 

 moved from the ordinary Acephala, to form a distinct class, were 

 such a division considered requisite. Their branchiae assume va- 

 rious forms, but are never divided into four leaflets; the shell is 



