176 



of the shell. In Gastrochana the animal has a pair of well-developed 

 symmetrical valves, allied to those of Saxicava ; teeth are dispensed with 

 in the hinge, but there is a well-defined ligament, and the animal encloses 

 itself within a club-shaped sheath, very subordinate in importance to 

 the shell. It is often incomplete, sometimes it is not present at all, and 

 it does not carry with it any very marked specific characters. In Clava- 

 gella the valves are of very rude construction ; not only the teeth but also 

 the ligament disappears, and only one of the valves is free, the other being 

 cemented to the wall of the burrow. The tube in this genus is of more 

 importance to the creature. It passes up through stone and coral and ex- 

 pands at the siphonal extremity like a flower forming, not unfrequently, a 

 succession of frills indicating periodical advancements of growth.* At the 

 chamber end it closes over the bivalve with a disk, and a few irregular 

 tubes are sometimes formed at the edge by corresponding processes in the 

 mantle of the animal, while the disk has a central slit indicating the posi- 

 tion of the animal's foot. In Aspergillum the bivalve is cast aside in a 

 very early stage of the animal's existence, and altogether imbedded in the 

 wall of a sheath of larger and still more elaborate construction. It exhibits 

 characters more obviously peculiar to the respective species, and the tubes 

 of the imbedded end are, by a more crowded development of filaments from 

 the mantle, elaborated into a rose, like the rose of a watering-pot. 



Six species of Clavagella have been observed, inhabiting the Mediter- 

 ranean, Australia, and the Pacific, dwelling at extremely variable depth. 

 " C. australis," remarks Mr. Broderip on observations made by Mr. 

 Cuming, " was so near the surface at low-water, that it was detected by its 

 ejection of the fluid ; C. elongata, from the nature of the coral in which it 

 is chambered, could not have been living far beneath the surface ; whereas 

 C. lata was dredged from a depth of sixty-six feet." 



Species. 



1. aperta, Sow. 3. balanorum, ScaccJd. 5. lata, Brod. 



2. australis, id. 4. elongata, Brod. 6. Melitensis, id. 



Figure. 



Clavagella apeuta. PL 45. Pig. 242. Shell, imbedded in limestone 

 rock, with a piece of the rock removed to show the free valve, the 



* "The frills are formed by the siphonal orifices when the animal continues elongating after 

 having fixed its valve and ceased to burrow ; or perhaps in some instances when it is compelled 

 to lengthen its tube upwards by the accumulation of sediment." — Woodward, Manual, p. 326. 



